This is a little late, but maybe it'll help someone else down the line. I called a Google rep and they said all the original buyer has to do is call in, give them their billing address, name, and the phone's IMEI. They'll confirm their information. Then the original buyer gives google your name, email, and billing address and they transfer the warranty to you.
All of this information was given to me under the assumption that I had received the phone as a gift. If you tell them you bought it off someone from Craigslist, I'm not sure if that would disqualify the warranty transfer..