If you are going to buy this game on ebay, or anywhere not retail or D2D, you need to specificly ask the seller if the game has been registered online or not. It can only be registered once and there is no going back, IC doesn't support used sales. So, if the seller says that "They don't know" don't buy it. If they say "yes" don't buy it. If they say it has not, and they will let you return it, then go for it. I also bought my copy from a co. in Jersy, but before I would pay I contacted them and got them to say that it was not registered, and if I got it and it didn't work then I could return it. It all worked out fine.
So, what should you do? Try what the ppl above said and make sure that have tried to reg. it correctly. Triple check the key, etc. If it is still no good, return the game, unless they said it was a used copy. Then its kinda your bad for not checking out your purchase.