Tell the seller they have to send you a cancellation request this is the ONLY way to do it so if the seller says you have to do it tell him to do the following...
Do exactly what you would to to file unpaid case except instead of choosing Unpaid case tell them to choose mutual cancellation which this will sen ou a request and on your end it is very important to accept or the seller will be out the seller fees ...
The request will come to you in your ebay messages