Buying AppleCare warranty in a different name than the original buyer's
I called Apple Support with just this question two years ago.  They seemed completely uninterested me showing that the machine had been properly transferred to me; my claim that I had it was good enough.  I bought AppleCare for a Macbook from Amazon.com and registered it with Apple received the final paperwork from them promptly, no further questions asked (this was in the US).

(FWIW, I did get a receipt and acknowledgment of the transfer of ownership to me from the seller in case it should ever become necessary, such as in an insurance claim).