Feeling cheated

The Ilanga Mall branch sold me the game "Call of duty black ops III". When we started installing the game at home we realized that the paper with the key was missing. We went back to the shop to get the paper with the number on. After a lot of searching for the info we got it. Back home started the process again just to get a message that this number is already in use. Back at the store for the third time, they just pulled up there shoulders and gave us our money back. How is this possible ??? Who loaded the game and how can they now sell it to a customer.