Apple is often the one that cancels it:
Seller on eBay has figured out the formula to generate AppleCare enrollment numbers that work. Seller sells you a random enrollment number that didn't come from Apple.
You enroll, and everything works great. Apple has no way of telling if you got that enrollment number from a bogus eBay seller, or a legit retail purchase.
Six months later, someone in an Apple Store buys an AppleCare box off of the shelf. They take it home, open it up, and try to enroll. Their enrollment fails, because the number in their box is the same as the bogus enrollment number you were sold.
Apple investigates. They'll ask both you and the other customer to provide proof-of-purchase, which may include the receipt and the box.
You were sold a bogus number from someone on eBay. You have nothing to send to Apple. Apple cancels your AppleCare and lets the legit purchaser of your number enroll his machine.
You can't change your seller feedback on eBay. The transaction is outside of the window of PayPal's dispute period. You *might* have a chance to dispute it with your credit card. And what sucks is that if this happens after one year from date of purchase of your iMac, even if you do get your money back, you can't buy AppleCare for your iMac (because the one-year mark has passed, and you can't add AppleCare after that).
So it's a gamble on eBay if you don't get the box.
