If someone paid for something, got a code, registered it, and then was told that they bought nothing with it with no recourse to the seller or to Apple, then yes, that is a lot more than a little bit of bad luck.
If you sold me a bad code and kept my money, you can bet I'd try to assault your business.
The problem of this thread is not that someone was sold a bad code on ebay, it's the assumption that ALL CODES ON EBAY ARE BAD! that this thread in title and for the most part portrays.
The reality is the OP got stung. Yep sorry for him.
But for the myriad others who have subsequently posted and said they have bought AppleCare codes on ebay, and have made valid claims with Apple without any problems whatsoever - it does seem very confusing and if anything misleading.
Just because one person gets scammed does not necessitate a blanket response for all. I have been scammed on ebay with a powermac G5 and lost a lot of money, I do not automatically assume everyone who buys similar is going to have the same problem. I have also bought all my applecare's in the last few years on ebay without hassle from the seller or Apple in the event of a claim, and the same for things such as PSN Sony Store coupons and Xbox Live Marketplace points.....