I've never really had a problem buying a defective game used, saved for a copy of Crazy Taxi on the Dreamcast. I already zone in on the higher quality cases when buying older games used, I just have to hope the clerk gives me a non-defective disc. By the way, does anyone else hate when they buy a game with an original case, and they give you the "greatest hits" version of the disc. That sort of thing always annoyed me.
But yeah, it's always a gamble buying older games used. Garage sales are no better, but a lot cheaper. So far my SNES collection is being obtained via the internet and handmedowns.