This is but one test, but Linus Media Group did some testing and found that mining GPUs worked just fine. Fan time and thermal pad degradation are about the only real detractors from buying a used GPU. https://www.youtube.com/watch?v=hKqVvXTanzI

Of course, if a card's running at thermal throttle for long periods of time, that has a chance to damage the card, but given that most cards are running underclocked, the likelihood that you'll run into that problem is low.