You have two options here.

1. Send the buyer a request for a mutual cancellation which they have to agree to in order for you to get your final value fees refunded and be able to re-list the item. With a mutual cancellation both parties can still leave feedback.

2. Wait exactly 4 days after the sale and open an unpaid item case. After another 4 days if payment hasn't been received you must close the case - then your final value fees will be refunded, you are free to re-list and the buyer will have an unpaid item strike recorded against their account. With this method neither party is able to leave any feedback and if the buyer happens to leave you negative feedback before the case is closed then eBay will remove it.

To activate either of these options use the drop down menu 'More Actions' next to the sold item and select 'Resolve a Problem'.