If you want a newer game, you're more likely to get it in good condition. The older it is, the more crappier a used copy will probably be.
If you really care about case condition, I'd recommend eBay or Goozex. eBay you can obviously see the condition. Goozex you generally get very good conditions. Everything I've gotten has been very good. A couple may have been a bit more beat up than others but they've been fine and I'm pretty picky, too.