Personnally I'd buy an ex-mining card, if it was 1-2 yr old. But only if I really trusted that it wasn't any older, so you'd pretty much have to buy from a friend or someone that could show u the receipt and the serial #s match etc.
I probably would have got a used 1080ti, if they weren't so expensive here in Canada. But on the other hand, I really really didn't want to spend that much on a used card from ebay, with no proof of it's usage history, just someone saying..it works.
No one's done a real study, with real statistics on this stuff. CPUs sure can last a long time at 24/7 usage. It's the external parts that fail 1st usually, and even the solder fails before the chips themselves a lot of times it seems.