I can see so many people abusing this to basically get free games, lol. Buy a game, rush through it within two days, return it for credit, use that credit to buy another game, rinse and repeat...
Well I was going to write this smart ass reply (obviously exaggerated):
This is Gamestop. The amount of store credit you get for returning a game probably isn't even enough to buy a pack of gum ... and if it is ... well guess what Gamestop doesn't even carry that!
...And then I realized that after reading the whole OP they actually give you all your money as credit (minus tax)
That's actually a pretty good deal. Almost makes me want to purchase Days Gone just to try it.