While you should definitely do your research and only buy from a trusted source, I don't know that I agree with you about not buying a card because it was mined on. (What about a card that was used for folding 24/7?)
Now, I wouldn't go out and pay $350 for a usedRX 480, which some feel like they can charge for a used card. But..,, good deals still exist. I recently saw a GTX 1070 for $300.., but wasn't lucky enough to be the guy to purchase it lol. I also just got a decent deal on another RX 470 for $180 (Which isn't much more then I was paying for them in the spring before the price hike).
I also personally own cards that have been mined on 24/7 for years, and still work fine (I've even got a 6950 that was used to mine bitcoin back when you could still do that on GPU, and I still play games on it lols). I've heard of people having to replace the fans before.., but honestly, that's both cheap and easy to do. As long as it seems like the person had a decent understanding of what they were doing, I'd buy a card that was mined.., Especially if it is a good price.
Also, as IR said, this applies mostly to the AMD cards (290x, 470, 480, 570, 580, and even Vega) and the mid-high end Nvidia cards (1060, 1070, 1080, and 1080ti.., yes, the 1080 cards are also used for mining). Unfortunately, that doesn't leave you with a whole lot to choose from.