Either issue a cancellation from Resolution Centre, bottom of any page or at "More Actions", far right of the item on your sold list. Ask the buyer to agree to the automatic email they'll receive then your selling fee will be refunded.
Or you can complete an Unpaid Item case which will put a non-payment strike on their account. More details here:
http://pages.ebay.co.uk/help/sell/unpaid-items.html
