Most transactions on eBay go smoothly, but sometimes there may be an issue. Most often it's because of a communication problem. First, contact the buyer to attempt to resolve it. You can also try to resolve it using the Dispute Console.
Just click on Dispute Console on the left-hand navigation bar in My eBay. This is the area to help you manage such things as unpaid items, items that weren't received or items that were not as described.