Decent cards are often sold due to upgrades, so they are probably good.  However, many cards are also sold because they don't function reliably [they may pass Furmark and other tests, but still fail in ways that annoy the hell out of you]; these are often cards that have suffered thermal stress, over voltage, whatever, usually as a result of gamer overclocking attempts.  Your risk to take.  I saw a post a couple of months ago that indicated that up to 40% of the 5970s coming off of Ebay were damaged in some way [whether they remained useful for mining when treated gently I do not know].

Good luck and buy used hardware carefully.