I agree with you but that's what they told me. There is a process you have to go through where you have to mail, email or fax the information for the new owner of the device so that the plan can be transferred over to them. I did that and they asked me for a copy of my receipt. I sent the paypal email confirmation and they rejected it because it was a non-Apple authorized reseller.
Now the good news! I just called Apple and spoke with someone in AppleCare protection plan administration. I explained my situation to him and he was confused because he said the only time they need a receipt is to confirm that you are indeed the owner of the plan or if you are asking for a refund on the plan. If you are asking for a refund, an ebay or paypal receipt is not acceptable; however, if you just want to transfer the plan no receipt is even required (they just want to make sure that you are the original owner of the plan). Anyway, he put me on hold for about a minute and he took care of the transfer for me and apologized for the confusion. He told me that the new owner of the plan would receive a certificate of coverage within 7-10 days and that the new owner's coverage was effective immediately. He also told me to just call the next time I wanted to transfer a plan and that's what I intend to do.