I don't wanna know what some of my games have been through. You have to remember that most people who buy games are not like us here on VGC, not at all. It's just a toy, or it's something to shut a kid up, or it's lived inside a bacteria frappe of a home where nobody cleans up ever. They just toss the disc somewhere, the cat pees on the cartridge, the cigarette gets put out on the console, the kid starts writing and drawing all over everything in the room, etc. This stuff is nothing special to them, it's money spent and forgotten about.
Then it gets traded into GameStop, who does not discriminate at all. Here comes the collector all oblivious and wide-eyed - that's the one I want!
Only the people who have their life in order, have their stuff in order... and that's not many people, actually. On occasion you might end up with something that came from someone with a clear mind and clean home, but I'm tellin' ya, it's not common.