If the seller is a UK business seller they are required by law to accept cancellations for online purchases. If this applies, ask them to send you a cancellation request, through the resolution centre. If the item has already been dispatched you can return it for a refund, but will probably have to pay the return postage.
Private sellers and sellers in other countries don't have to accept cancellations. You can ask, but they don't have to agree. If a buyer doesn't pay, they will open an unpaid item case to get their selling fee refunded. EBay will place an unpaid item strike on the buyer's account if they still don't pay.
Contact thre seller explain the error ask them to send you a cancellation request.
You can also use the Cancel Order facility from More Actions right of the item purcchased, but the seller does not have to accept this. If they do not, you face an unpaid item case being opened so the seller can retrieve their seller fee and be able to relist their item.
As coins cannot be sold in the New Managed Payments it is unlikely that your seller will be a Business Seller,so unlikely to agree to cancel. Unpaid case is far better for the seller.
