I haven't had any terrible experiences with GameStop. I know what I'm looking for, go in, look for it, buy it, and then get out as fast as I can. I guess that's when the horror stories start flooding my mind.
When I bought Picross 3D a few months back, the cashier actually told me she thought it was good and that she put a good amount of time into it. Then about a month back, some woman was confused about the 3DS (thinking it was discontinued), and the clerk explained that it was just a price drop. Cleared things up there.
But yeah, taking 5 dollars of a "used" new game is probably not very nice.