I just found this in the GoPro Forum, thought it would be useful to add to this thread and could help others out in the future.
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If you're not in a rush, it's rumored that GoPro is going to be releasing a budget version of the Hero5/6 cameras at the end of the month. I'd personally wait for that instead of going the used route. If you need something right away, then the Session is the next best budget camera. Target, Walmart, and B&H are still selling them. For just fun shooting that is easy and convenient, the Session is the best camera IMO. It's small, discrete, super durable, and takes good video and audio. You don't need 4K unless you are doing editing on your computer.
Cameras to avoid:
Hero3 Black/Silver/White: Poor battery life, limited resolution/frame rates, old style Protune offering fewer options and requires color grading in post editing.
Hero3+ Silver/White: Basically just the Hero3 Black and Silver. (Hero3+ Black is OK)
Hero, Hero+, Hero+LCD: These were basically throw away cameras permanently infused into the waterproof housing killing any hope of decent audio. Not terrible but once you have an issue or problem there is absolutely nothing you can do. Scratch the housing lens and that's it. A common reported problem was when the housing mounting arms would break making the camera unable to be mounted. Since the camera was stuck in the housing, you couldn't replace it.
First release Hero5 Session (used models only): The first batch of Hero5 Session cameras had a higher rate of complaints from freezing/overheating. If someone is selling their Hero5 Session, chances are they have one of these.
Top to Bottom Best cameras:
Hero6 Black - far and away the best GoPro camera. If you get this camera you will use this camera in ways you aren't thinking of now. Beautiful picture, amazing stabilization, great photos, time laps, night photos, night laps, ZOOM, fast video transfer to mobile devices, Quik Story compatible, Auto cloud uploading, very good low light (with latest update), wide range of resolution/frame rate possibilities, voice activation, good audio, waterproof without a case, GPS, selective light metering, Auto burst photo mode, Auto shutter with ISO 100, and with a clear, easy to use rear LCD. You pay more, but you end up getting a camera you can actually use for more than just your ski trips.
Hero5 Black - Similar to Hero6 Black with many of the same feautres, BUT... Video quality is not as good, less resolution/frame rate options, 2 axis stabilization that is good for shake but nothing close to the Hero6, no Auto Shutter with ISO100, no Auto Burst mode, fewer voice commands, slower Wi-Fi transfer of media, slightly less responsive LCD touch screen, poorer audio, no ZOOM, and some reported complaints of waterproofing without case giving out.
Hero5 Session - Great little camera that is durable, discrete, and gives wide range of resolutions/frame rates including 4K/30. Earlier batches seemed to have more reported issues but later runs seem fine. I use this camera almost as much as my Hero6 Black. Great waterproofing and nearly indestructible. It also works with QuickStory for the mobile app and has Auto Cloud updating.
From here it depends on your wants/needs. For a simple no frills camera that is durable and gets good audio and video, the Session (Hero4) is great and my number 4 favorite and used camera.
For very good video and audio with the camera out of the housing, not waterproof and vulnerable to damage, then the Hero4 Black. Great camera if you know how to use Protune and use outside the waterproof housing or in the frame. Inside the waterproof housing audio is unusable.
Hero4 Silver: Basically a Hero3+ Black with a rear LCD. Same issue of needing to be out of the housing for decent audio. Most frequent complaint is a helicopter "thumping" noise that comes through in the video recordings when Wi-Fi is used.
The Hero3+ Black: A great camera for it's time but there are much better options at around the same price. You have to use it in a housing if you want to keep it safe, which completely ruins audio and the video quality isn't as good as the Hero4 Black.
Mostly, when looking on sites like Craigslist you are going to see cameras from the do not buy list. There is a reason for this. Most of the cameras on the Best cameras list can be found, but not at cheap prices. There is a reason for this as well.
If you want to keep your camera safe, only use GoPro mounts and accessories. The 3rd party brands are cheap junk and break easily.
Hope this helps.