The few investigations have generally found mining cards are perfectly fine and if anything above-average in used quality. Because yes the primary damage is from thermal cycles, not just being on. And since mining is a constant load & constant environment, it has very few thermal cycles. And miners primarily care about efficiency, so they're power-limiting them[1] which further reduces strain on the cards.
Fans might be closer to death than a typical used card would be, but on pretty much any GPU those are cheap & easy to replace. That's really the only "risk"
1: And before someone inevitable scoffs at this, it's common enough that this is built into tools like nicehash. It's literally "click 1 button" territory, not exactly something a miner wouldn't attempt.
Yup, assuming it works and checks out the first 24 hours I have it.
Solid state electronics are pretty robust. If they survive whatever torture you put them through plus being shipped to my door, chances are it will work just fine. In my experience there are some problematic GPU models that are more prone to early failure under heavy use (HD6990 I'm looking at you!), but they are more rare than legend may lead folks to believe.
At an appropriate discount it's far cheaper to just buy these at pennies on the dollar and just be alright with self-insuring for a rather rare early failure. When you can buy 2 or 3 for the price of new, it starts to get rather compelling.
If I can inspect a card in person, I would be plenty happy to buy an ex-mining card, but most of these are going to be sold online sight-unseen. This isn't a problem exclusive to mining cards though; people have been selling damaged GPUs as 'working' for years.
In some ways, letting a GPU run at a constant load (and thus constant temperature) is less damaging than a typical gaming workload I imagine, where the temperature fluctuates often. In other ways it probably isn't (e.g. atoms of the chip 'migrating' over time with the constant electron flow, though I have no idea how much of an issue that is with modern chips).