One of the driving forces of Gamestop is also the fact a lot of kids/teens don't have jobs, so a lot of their money for new games come from trade ins. Gamestop is the only viable place to trade them in without dealing with all kinds of bank accounts, paypal, etc, so its easiest for teens to do it there.
There are other used game stores
**** EB games was much better than GameStop but people supported GS until they got too big and just bought EB out
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I haven't played The Wind Waker-LinkMarioSamus