MaxxxModelz raised a good point, but there's no way to know if a card was used for crypto currency mining. It would be extremely rare to find a seller willing to admit that if he's trying to get rid of the card. A good rule of thumb might be to just assume any new generation card you see for sale on eBay is suspect and probably used for crypto currency mining. Especially now that AMD and NVidia may come out with new cards specifically for mining. You will likely see a ton of recent gen cards flooding the market, and they will all be in very poor condition internally.

https://www.engadget.com/2017/06/27/nvidia-and-amd-currency-mining-cards/

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