WOTLK retail box bought but key not working.

I went out and bought a retail box set for WOTLK and tried to enter the key to allow me to use it online but every time it tells me there is an error. I have tried several times and checked the key over and over to make sure I have entered it correctly. Still no dice.

I now have the box with the software which is already out of date so I'd have to patch it anyway but no way to use it online. Game software shop won't take the software back because people buy the game, use the key and then return the box. I've even tried asking them to load WOW and let me try it on their PC to prove it doesn't work at the shop but they won't have it. It seems I now have a useless DVD that I've paid for but can't use and can't even return to get my money back.