Some idiot bought it from Redbox and traded it to Gamestop. It isn't very likely, but it is possible.
This, its actually very possible. I already posted one of the reasons someone would do it.
When you rent from Red box if you don't return it after so many days you buy it and they charge you the full amount. Its a price cut off, with movies once you hit 20 dollars they charge you whats left after the initial rental and with games its 50 bucks.
As I mentioned before I know someone that used a new game at Redbox to get enough credit to get one that he wanted that day. He paid for it in the end, but he got the game that day.
There is no reason for Gamestop to not take the discs. I am sure there are metric tons of legitimately purchased games and movies out there from Red box. Whether the person forgot to take it back or wanted to keep.