http://www.walmart.com/cp/Pre-Owned-Games-Learn-More/1056244 Same Return Policy as for New Games
If you were shipped a video game that is not working properly, you may return the defective game within 21 days of receipt.
All pre-owned video games purchased from Walmart.com must be returned unopened either to a store or by mail within 90 days of receiving it.
For additional information, please view our full Return Policy.
So basically once you open the shrink wrap on the used game they could refuse the return the same way if you tried returning new unopened game that you opened