Return policy of all new unopened items should be 30 days in all stores. Or atleast most stores.
For example. I purchased Dragon Warrior 3 for the Gameboy Color for $35. I couldn't find a new copy anywhere. Then a week after I purchased it, it was in stock for $20 new. I complained how I just purchased it used, and at first they were like, well prices drop, etc, etc, despite that I told him I purchased it a little over a week ago. So I talked to the manager who saw that and then took the used copy as a return and gave me the new copy.
For used games Gamestop only has a seven day return policy, but thankfully there are some people who put customer service above screwing people over and also had some common sense.