The first thing you need to do is send the buyer an invoice for repayment (you can create an invoice through Paypal and send it to them).
Then you need to advise the buyer through email and (preferably) registered post as well that they can return the item - provide a means or method for them to do this - or payment is expected by X date. If payment is not made legal proceedings will begin, commencing with a report to ACORN (a federal agency dealing specifically with online fraud, who pass on the details of reports to the police local to the reportee - in other words, the buyer's local police will be notified that someone in their jurisdiction, if you will, is illegally in possession of goods).
That might prompt them into action.
