Also Hypernova is wrong. You agreed to a legal transaction by offering it on e-bay. It's a contract ! Technically you can't get away with it. This is for seller and buyer alike. Read the fine print.
Ooooooo!

Like the fine print's ever stopped anyone.

I don't give that advice to anyone to ignore a transaction for any dumb reason. I only gave the advice because let's face it, once the address was changed, everyone's spidey-sense was tingling. He originally wanted to get out of the transaction. Via a courteous email and a full refund, this is easily accomplished. eBay has options to cancel transactions.
I just had to opt out of a transaction to Brazil (different one) because Paypal flagged the transaction and held it. About a week later, the guy deactivates his account, and the following day, emails me about hoping the payment goes through. Once he left eBay, I contacted Paypal, and had them refund his money that was held, and told the buyer it was over.
Now it's not the same situation, but similar results.