Only the seller can cancel an order but you have a couple options. If this was for an auction or a item you accepted an offer on and the buyer has not paid you can simply wait 4 days and cancel the order for non-payment and the buyer will receive a strike. If this is for an order where the buyer has paid and the item is waiting for you to ship you can simply go to your orders page, find the item and there is a drop down button, select cancel order and select buyer requested. EBay will refund the buyer and your fees except for the $.30 cents.
Non-payment strikes are the best way to handle non-payers since the strikes help sellers to block potential buyers who have a habit of canceling orders.
