Why do people sell their second games to stores like gamestop instead of selling them directly to other consumers?

[Dinges]

I live in the Netherlands where we don't have a gamestop. We have some other smaller stores where you can sell your second hand games to, but not many people do that. If I sell my games, or if I want to buy a second hand game, I go to a website called marktplaats (=marketplace). It's a bit the same as ebay, but you don't have to pay the website anything and your advertisement will be available on the website for 28 days for free. In this way you can cut the "middle man" and  sell directly to other people without some big company who wants a piece of the pie. I don't understand why people sell their games to places like gamestop when they're "screwing" their custumers over by asking a ridiculous prices. Why do people sell their used games to stores that give them less money than if some consumer would buy the game directly from them and why do people buy these ridiculously overpriced second hand games?