SRV1981

I've had some great bits of luck recently by using search terms that reflect what a person who didn't know what they had would use as a listing title - 'camera light' and so on... then sorting by Ending Soonest.

I'd try a range of things, and I'm sure others will have more to add, but I would:
Check the camera physically to make sure the screen works, buttons, EVF, etc I'd update the firmware straight away to the latest Put on several lenses and test that they're recognised correctly and the AF and OIS are working After putting in a new battery and formatting a memory card in it, I'd pick the best quality normal mode (24/25/30p) and do a long recording on it of something that has a lot of movement in it (a great test is putting three still images on a timeline each at 1 frame and then loop the video)..  if it records without issue for 20+ minutes without anything odd happening that's good, but if you have time I'd test until the card fills up or the battery dies I'd also do the same but on the highest frame rate mode Check the files are playable in the camera and work on the computer If it passes all of the above then it's unlikely that it has some lingering issue that isn't also present on new copies as well.

B&H and Adorama are my most trusted vendors for used stuff. Fun thing about Adorama is that almost all their used stuff on their site is also listed in their eBay store with a “Make offer” option so you’re able to buy things under list price. They’re pretty good about actually accepting offers, too. 

I’ve had very good luck buying and selling with MPB in the past, but you need to reallyyyy stress test stuff from them IMO. I’ve returned a couple items that I didn’t feel were listed correctly. Like, things I bought at “Excellent” that I would characterize as “Poor” or “Acceptable”. Still, there are some great deals there.

Aside from launch day preorders like the BMPCC4K, I buy almost all my cameras used from major retailers. You’d be amazed how many cameras get returned just a couple weeks after launch. Like new condition for 10-15% off. I stress test them quickly just to make sure there isn’t some defect. 
Typically, anything rated 9 or higher from B&H is indistinguishable from brand new.

 

About half my cameras were second hand, with a couple of them likely having a lot more than two previous owners, but haven't had any issues with mine.
If you're concerned about overheating, get a camera with a fan.  A fan is the difference between a camera overheating in air-conditioning in under 45 minutes vs a camera recording for 24 hours in a race car at 120F / 48C.

I've purchased some new and some used. If you're super lucky, you can buy new for used prices. IMO, that is the time to buy. For Panasonic, it's during Black Friday. A few years back, I got the Olympus E-P7 with the kit lens and the Olympus 75mm f/1.8 for 899 euros. That's probably the best deal I've ever found for new. In France, you have leboncoin, which is very good too with some amazing deals from people who just want to unload material or just don't understand what they have. When you buy, you have the choice of sending it back. If you know your cameras, you can thoroughly test it. Buying off Ebay is an option too, but you never seem to find good deals anymore (I gave up). MBP is also a great choice, but they're too expensive for some cameras and lenses.

If you are really worried about that, buy it and stress-test it quickly so that you can return ASAP it if you're not happy.
But if long recording times in hot environments are important to you, then you should be looking at a camera with a fan anyway (so you don't get distracted from the enjoyable creative stuff by worrying about overheating). Otherwise avoid leaving the camera out in hot direct sun/put a sunshade over it/put a white or reflective cloth over it. And turn it off when not using it, so it's not already hot before you press the record button.

I've owned many cameras (too many) and i think I've only ever bought 2 from new. Thing is, there are so few moving parts on cameras these days that there's not a huge amount that can go wrong in normal use. That said, unless an eBay or similar deal is just too good to pass up, you are safer doing it through a dealer, as @ac6000cw says.

If you want to minimise the risk buying used, buy the body from a major dealer who will provide a decent length warranty and allow you to return it if you're not happy with it e.g. in the UK, Wex offer a 45-day return period and provide 12-month warranties on used gear. MPB offer 6-month used warranties, as do some of the other major UK dealers.
If you want long warranties, buy new and then buy an extended warranty from the manufacturer, or wait until the manufacturer offers a free extended warranty as a sales promotion on new bodies.

I also buy most of my gear used; I did buy my Sony A7iii new, mainly for the warranty. For Blackmagic Design cameras I think it's best to buy used because BMD's quality assurance is so poor that you may need to buy several new cameras to find one that works. The two BMD cameras I bought new were defective out of the box and had to be returned; the used one I bought on eBay is perfect. People who buy a defective camera will likely return it to exchange, so you'll probably have better luck buying a used one.

according to my experience, this is true. 

Another thought and that is have you considered a full frame lens?
I wasn’t going to, but testing the photo side of my kit on 2x portrait shoots, I decided I did have a hole in my lineup...so bought another Tamron E Mount lens, the current version of the 28-75mm f2.8
Now I suspect in FF flavour you are going to call that too short, but on your APSC camera, it’s going to be a 42-112/3 equivalent.
Still too short, IDK, only you can answer that, but it weighs basically the same as the APSC 17-70, so thought it was worth mentioning?
 
 

Would easily be my first choice for APSC with a real world 28-105mm focal length and constant f2.8 aperture.

I just bought the Sigma 18-50mm f/2.8 for my Fujifilm, but it's also available in E-Mount.
I'd rather sacrifice some range and have a smaller lens.
Else there is also the Tamron 17-70mm f/2.8

I had the 18-135mm for a bit. It's a very compact and lightweight lens. Good range. Great AF. The weakness is obviously the f/5.6 aperture on the long end. It's fine for outdoors stuff, but I found myself really wishing I had an extra stop of light pretty consistently indoors. On my full frame cams, the 24-105mm f/4 is my workhorse. It strikes the right balance of range, aperture, and performance. I'd imagine the 18-105mm would be a similar experience on APS-C.

I agree the 18-105 is a really versatile lens: power zoom and near par focal. A little deficient on the wide end, but now there is a perfect complement: the Sony E PZ 10-20mm constant f4.
Shooting Sony APS-C has real advantages in terms of video-advantaged lenses.

I'll answer it for you and I've used the A7s3 and FX6 but not the A7IV.
Both cameras have heavily processed shadows that ruin the image. The A7s3 and FX3 are superior because you can shoot RAW externally and bypass the processing. Dynamic range isn't useful when it's being ruined by noise reduction.
The fx30 has much less processing at the lower native ISOs and would be my choice over either of those cameras. Or get a Nikon Z8.

Perhaps the premise of the question would be interpreted differently if the two cameras were more different to each other.  For example, if I asked the same about the GH5 vs GH6, then there would be things to talk about, and spending time looking at the various specs and performance would be worthwhile.  It's also helped by the likelihood that I'd own the GH5 and be questioning an upgrade - so it's a question that would have real-world implications and would be of value to discuss, rather than a purely theoretical question.
In the case of the A7S3 vs A74, the strengths are very similar or don't really matter (both have enough DR for almost anything you might want to shoot) and the weaknesses are similar too.

All good points, but perhaps most significantly, it has a cooling fan.....  in something approaching the size of an action camera.

Sony, etc, have no excuses.

Makes sense. I think the main differences between these Sony cameras are rolling shutter and the ability or lack of ability to record RAW if that is a feature you want to use. Dynamic range is all very similiar in latitude tests and imatest measurements. 

My pick for Sony is the FX30 as it is the cheapest and has a damn good image and a lot of cheap 3rd party lenses for it too. 

I'm confused...  you obviously don't want one, so why are you in this thread about camcorders arguing that no-one wants them?
I'm not interested in buying any cameras that exist right now, but I'm not sitting in all the threads arguing with people about them.

You could check the cined lab tests for both.  If you do, the a7 iv seems to have better DR (though much worse rolling shutter).

It's also worth pointing out that the film you posted could have been shot with almost any camera just by controlling the ratio on set with lighting.  I'm not being dramatic here - you could literally shoot that with the GH1 or my GF3 (of course the resolution and bitrates would suffer..) but the contrast could be replicated.

I think comparisons like this are a journey into very tricky territory.
I'm not saying that DR doesn't matter, or that the A74 doesn't have more than the FX3, but I wonder what situations this will really make a meaningful difference in.
12.9 vs 12.4 isn't that much of a difference, especially once you've got this much DR already.  I like to think about this in terms of "what shots can you get with one and not the other?".  I have spoken a lot about DR in my work, but that's because I tend to regularly be shooting people in front of a sunset, and I want the persons face and also not to clip the sunset behind them.  I think I could get that shot with either of these cameras.
Another shot is of people sitting by a fire, and wanting to show the surroundings as well as not clip the fire (white fire looks odd but you can clip little bits of it as they end up looking like fire highlights).  This isn't a shot I do so I'm not sure if this needs more DR than the A73.
You also need the skill to pull the shadows out in post.  I don't mean just raising the shadows slider either, I mean having the skill to match a shot with (let's say) 12.9 stops of DR with the previous and next shots which are likely to have dramatically less DR.  Imagine that the surrounding shots are of people sitting around the fire - they might have half the DR of the previous shot.  When you have the contrast / curves / LGG wheels / etc cranked one way for one shot, then cranked the other way for the next shot, will all the colours remain similar?  Will the contrast seem natural?  
Colour grading software is getting better but did you know that the normal colour grading controls were developed with algorithms that were simple for computers to do (to reduce processor requirements) rather than looking good?  This often doesn't matter when you're only adjusting things a little bit, but large changes will ruthlessly reveal the weaknesses.  Things like the Resolve HDR Palette were designed to be perceptually uniform, but are you grading in Resolve using this panel?
You also have to consider the other implications of a choice between the FX3 and A74, for example.  Comparing DR is great but it's an "everything else being equal" comparison, but of course, everything else isn't equal.  The extra DR might come at the cost of something else that is more meaningful to what you're doing.  Film-making doing anything other than shooting still-life images on a tripod inside (where everything is controlled and you have all the time in the world) is an exercise in having less time and attention than you'd want to, so you're always trying to work out what is most important and paying attention to that.  I don't know about you, but I routinely look at the footage I have shot and see all kinds of things that I should have done differently - but then I have to remind myself that I was walking down a staircase holding the camera with one hand and the railing with the other and trying not be steady and use the ninja walk while keeping the right focus distance to my subject and keep the nice framing and also not hit my head etc etc etc.  I didn't have mental capacity to do anything else, and if I had paid attention to that other thing then maybe I would have screwed up the shot entirely, etc.
You have to look at the total situation and review what the requirements are for you in your situation and think through what implications you'll have of swapping from one camera to another and the effect on your viewers as they are watching the final edit.  Everything else other than how your viewers feel is a means to an end.

In digital, highlights don't roll-off - they clip faster than a Ferrari without breaks.  The roll-off is applied as part of the processing that occurs after the image is read from the sensor.
In terms of DR, you have to find a source that has tested both with the same methodology.  Luckily, CineD is one such source:
https://www.cined.com/sony-a7-iv-lab-test-rolling-shutter-dynamic-range-and-latitude/
https://www.cined.com/sony-a7s-iii-lab-test-does-it-live-up-to-the-hype/