I find used games mostly awful, because of the condition. If you buy one from a rental store, you never get the proper manual and if it's a cartridge, it'll have vivid marking and void stickers on it. If it's a disc, it might have scratches (if you are lucky, a disc guard is stuck on). The case won't be the proper one, but if it is, it might be altered...
Preowned Games from EB and other stores like The Gamesman and Central Park are likely to have scratches or missing manuals, making them useless.
I bought two GCN games on a national auction website and my the system at a rental store for $100NZ, with the Cube, the leads, two memory cards, three controllers, but no games (I bought them after a while; I recently got Metroid Prime 2: Echoes).
So basically, I buy good condition stuff, but only go online if there is a game in perfect condition.