I've only ever purchased a few used games online. At least at GameStop you have the ability to look at the back of the disc first. After seeing the way some of my friends and family take care of their discs (music, movies, games, whatever), I'm more concerned than ever ability getting stuff second hand. Of course, NetFlix rentals are just as bad. It's not often that I get an particularly popular film, but when I do, I'm almost guaranteed that it won't play 100% correctly.
If cleaning them doesn't help, you might consider getting your a disc doctor. It'll actually shave a thin layer off the underside of the disc. It removes the scratches quickly and leaves the lower data intact. Of course, you wouldn't want to do it more than one or twice to any particular disc... lest you destroy it.
