Actually that's not a logical conclusion to draw. Often these "code only" purchases register just fine, and the buyer leaves positive feedback. Then, a few months later Apple catches on to the seller's scam and requests proof of purchase from the buyer. Since all the buyer has is an emailed code, their Applecare is voided and it's too late to seek any recourse from ebay or PayPal. To add insult to injury, the buyer's positive feedback remains to lure other suckers in.
In short OP, don't buy Applecare off ebay unless it is a boxed version. Otherwise you have no proof of purchase, and Apple can invalidate your Applecare. It has happened to several.
There's a mile-long thread here about it. This topic comes up every few weeks so there's more where that came from if you care to Mroogle it.