Used usually ebay, newer games but still used on forums. It's not odd to see a almost brand new game used that someone did not like selling for $30 on a forum where on ebay they are still going for $50 +
i would rather buy my used games from the forum(here) or on ebay. i've never had 1 game stall that i bought used from either.
my friend bought gta4 used at FYE and that shit had a SLOW *** load time and stalled constantly, he got fed up and returned it and just bought a new one.
i bought a used tony hawk game from FYE as well and it stalled early in the game as well. had to return it and i got the copy replaced.