don't do it
Don't do it, it's a scam. I had someone try the same thing with me when I tried to sell my 3g on ebay. It sounded shady so I googled it and found that a legit serial # can somehow be transferred to a stolen iphone in order to activate it. I don't know the technical stuff on how it works, but it is a scam. Within an hour ebay contacted me because I had responded to the persons email, saying that any correspondence with his account was suspect because the account had been compromised.