I bought a used version of Phantasy Star Online when I was a kid and someone had thrown up in the case. The game was fine, but the left side of the case and the manual were just disgusting. How the hell did gamestop let me buy that?!
But somehow, I wasn't traumatized that much, and I still buy used if the price is low enough. This year, I've purchased Bayonetta, No More Heroes 1/2, and REmake used and they all work great/are like new.