usually the guest account have strange number letter sequence and a lot of them. He will only be able to leave feedback from the account he purchased on so don't worry about the second account and if that account was a guest account then he won't be able to leave any feedback.

 The problem is, I hope you sent the item to the paypal address associated with the first account.

If you did and you have tracking then to prove it then I would simply tell him that you can't issue a refund as the item has been sent and is in transit. if you feel like refunding him you can always tell him that oncew he recieves the item he can send it back at his expense and you will refund the initial price and postage.

You don't have to though as it's a change of mind.