MacBook Pro :: Registering MBP Without Proof Of Purchase And Applecare?

If one buys a brand new sealed macbook pro from a private seller off craigslist/ebay/kijiji without a receipt can they register it with Apple? I know if one checks the serial number it will tell if there is warranty on the machine and an estimated purchase date. However, I was told by Apple that I would not be able to purchase and attach applecare to the machine because there was no prooof that the machine was infact purchased by someone - but that the machine does have one year warranty from the estimated purchase date.

Wondering if anyone has encountered such a situation where they bought a machine privately and then wanted to add applecare to it within the standard one year warranty period and was denied by Apple because there was no proof of purchase for the hardware. Or was the Apple rep just misinformed - if a machine is under one year warranty then one can purchase and apply applecare anytime within the one year.

Wondering if buy a machine and then 3 months down the road decide to buy applecare, has anybody been denied adding that applecare even though the machine is covered under the one year warranty. Also has anyone resistered hardware in their name without proof of purchase other than an ebay/craigslist printout?