Actually, the poster was quite correct. That scenario has been happening to MANY people buying those on eBay. Maybe you bought some that were legit that way, but scammers have found ways to rip people off right on eBay with AppleCare. Even sellers with hundreds of positive feedback have been involved as people leave positive when they initially receive the code and register it. Later, they find out their purchase code and coverage was cancelled. Also, some have found ways to break the code and determine what codes are using the right mathematical formulas.
There are a lot of problems if you buy AppleCare and don't get the box with the code. I believe that you received some fine, but it has become too big of a problem, and I wouldn't buy on eBay unless I received a box with an unopened code.