On e the listing has ended you can't cancel a purchase. You have to ask the seller if he would offer you a mutual cancellation request. This would relieve you of the obligation to pay for the item. If the seller chooses he can refuse to do this and open an unpaid item dispute against you.
If the dispute is opened and you don't pay you will get a strike and can end up on the blocked bidder lists of thousands of sellers who block members with 2 or more strikes in the last 12 months.
You should 't buy anything in eBay unless you are fully prepared to follow through with the transaction.
