If they won the item then you open a non paying bidder dispute. They are time wasters and it is their responsibility to make sure no one uses their account with authorisation
Open a non paying bidder dispute after four days and close after another four days. You get your fees back and the buyer gets a strike and cannot leave feedback.
At the side of the item click More actions then click Resolve a problem to open the dispute.
You should always sort out one sale before trying relisting or a second chance offer as you could potentially end up with two buyers and one item.
If they both paid you could end up with a non performing seller strike
Never do a mutual cancellation for a non payer as they could decline it and you would still have the fees to pay and they could leave feedback.
Non payers are time wasters.
Do not be tempted to leave positive feedback with a negative comment. This can be reported and removed and you will get a black mark on your account
Most people have their preferences set to block bidders with two strikes.
http://pages.ebay.co.uk/help/sell/block_bidders.html