If the seller has opened an unpaid item claim against you ask him if he'd be willing to cancel the transaction. You stated that you've tried to contact him but got no response on several occasions.
If this is the case and he refuses to communicate or agree to a cancellation at least you've covered your own backside.
If he decides to proceed the lack of communication on his part won't look good.
If it's his policy not to ship to a P.O. box but doesn't state this anywhere in the listing and you have indeed attempted to work it out then he hasn't got much of a case I'd say.
