If your listing has been sold and you are no longer able to fulfill the order or have come to an agreement with the Buyer to cancel the order. You can go to that particular sold listing and press the "I need to cancel the order" button.
Please note that if the listing has already been marked as Shipped, then this option will no longer present itself. If you'd still like to cancel an order after this, please contact Support and we can assist you in doing so.
To avoid issues, if your item is no longer available, please make sure to unlist your item.