Are you the buyer or the seller?
If you are the seller, open an Unpaid Item Dispute.
http://resolutioncenter.ebay.ca/
If the buyer does not pay, you get your fees back, and the buyer gets a Strike which makes it harder for him to bid or buy on eBay in fulture.
If you are the buyer, contact the seller from the listing description, Contact Seller, and request a Mutual Cancellation. Do not agree to an MC unless your payment has been refunded.