Selling my iPhone: what about warranty?

Hi, i am planning to sell my iPhone 4.

First of all, i have bought it not as a private customer, but as a commercial one: furthermore, i have purchased also the Apple Care, which makes the warranty last till end of December 2012(this is what i get by putting the iphone serial number in->https://selfsolve.apple.com/agreementWarrantyDynamic.do)

So, this is my question: at the moment of the sale, do I have to transfer the warranty to the buyer? If so, how?

Or it's enough for the buyer to put the serial number on the website i wrote above, if he had to have any problems with the iPhone?

Thanks in advance.