I don't know if you as a buyer can reverse a transaction once money has been sent; otherwise, there could be fraud with people paying for items and then taking the money back once the item has been shipped. I think the seller has to refund the payment.
If the payment has gone to an address & is not claimed, then you as the buyer CAN cancel the transaction. If it HAS been claimed, then the seller has to refund it.
I've had this happen to me and I'm always annoyed because I feel it is the SELLER'S responsibility to accurately list their email address with paypal when they list the item... Your email changes, then change the listing. It's VERY easy to add an email address to paypal if that's the issue...