I'm not from the US. My profile flag is fake. I only use it because Argentina is my favorite football team.
Essentially Gamestop serves as a "middleman". They "sell" the game for you. It's like a pawn shop. You'll get a lower price than regular sale price, but you'll be guaranteed money. You go to a flea market or Ebay and you won't have the reputation behind you of Gamestop. You buy a used game at Gamestop and you know it's A) legitimate, B) will work, and C) you're guaranteed a refund if it doesn't pass these criteria. People are much more hesitant when buying on Ebay or at a flea market because there's none of these guarantees.
You could easily post all these games online and have half of them not sold, and wait a long time just getting rid of them all. Time is money they say, so selling to Gamestop is just better in the end.
The problem is that it is nearly impossible to find a mint copy of a game at any game store. Many times you can't find a copy that is in an original box even. Online you can find the condition/price that suits you (while sitting around in your underwear I might add). Also collectors (especially of RPGs) like to find copies of games for systems that have discontinued support at most game stores. Lastly, there are many times where you can get games cheaper on eBay, etc. than at a game store which benefits both parties. True, people can and do get screwed so that is why you need to look at people's feedback.
Mostly this all boils down to settling for less because you are too lazy, naive, and/or wasteful. I know this runs higher in the US than probably anywhere else because of the want for instant gratification here.
Calling people lazy, naive, and wasteful is judgmental. By calling your opinion fact, you neglect absolutely everything everybody else said. To summarize:I'm not judging anyone. I'm just stating the facts.