I bought it on eBay for $130, and it registered with Apple - no problem. I do have all my receipts including the purchase receipt from Apple for the MBP, and the ebay/paypal receipt from the seller for the Applecare.
You won't find it listed that low, but if you search some of the sellers on eBay who have solid feedback (99+%) and hundreds or thousands of transactions, and their feedback comments are specifically about Applecare sales, then I'd feel comfortable doing it. Use paypal, and you have some protection.
Some sellers have two different 'buy it now!' prices, and the higher one might have a "make an offer" button. If the seller is highly rated, try to offer a low price. If it doesn't work, just up the offer. Once you hit the price they'll sell at, you'll automatically get an acceptance of your offer. So you can fish for their bottom price that way. I just offered $130, and when it was accepted in 3 seconds, I don't know if I could have tried a lower offer at first, but I just paid via paypal, and in an hour or less I had my registration info. It worked at Apple support, and the next morning I had my confirmation of coverage from Apple.