There are several ways that eBay users can get these, only one of which does not cause a scam:
1. Bought from Apple/Dealer in a box. Lowest price possible is ~$150.
2. Code is generated using a keygen (No box, code-only deals)
3. Bought using a stolen credit card.
4. Bought, then returned after the 45-day PayPal complaint deadline.
Basically, 4 things can happen to you when you buy off eBay:
1. You bought a BNIB AppleCare which includes a box. You have the serial number on the outside of the box if Apple questions the validity of your AppleCare, and you can provide it to them to confirm authenticity.
2. You bought a code-only AppleCare, which does not have a box. It was probably acquired fraudulently. Apple doesn't discover it, and you get the full coverage, maybe even a repair or two. Consider yourself lucky.
3. Apple discovers you have a fraudulent, generated AppleCare because someone else with proof tried to register the same code. They ask you for the serial number from the box or original proof of purchase. You can't provide either (eBay/PayPal receipt does not count). Your AppleCare gets invalidated. If you're outside the 1-year warranty, you're SOL. Otherwise, you're out what you paid for the AC, and have to buy another.
4. Your AppleCare is returned by the seller, or the person whose credit card was stolen disputed the charge. Apple removes the AC without notice from the account. Same as #3 in terms of final outcome.
Basically, there's a ton that can go wrong, and very little that can go right. If you want to be safer when buying off of eBay, get it new in a box, and make sure you get the box. The serial number on the outside is proof enough that you bought it.
However, if you buy it off of eBay, do note that you can never transfer the plan to another person. That requires original proof of purchase, which you don't have.
In order of value, you would have:
eBay Code Only > eBay Box > Apple Reseller w/ Educational Discount > Apple Reseller Normal Price.
And in order of safety:
Apple Store > Apple Reseller ~= eBay Box >>>>> eBay Code Only
Buying it from the Apple Store means they can use their internal systems to check the purchase of the item, so they would know you bought it from the Apple Store. Apple Reseller and eBay Box are about the same, with the Reseller having a slight advantage because you also would have a receipt to show. And they're all way above eBay Code Only because you can verify your purchase.