Orders can be cancelled by mutual agreement. Above the message box on your order page (My Orders or Manage Sales) there's a link that says "Order Issue".
If you click the Order Issue link, you will see an option to request to cancel:
You can also suggest a cancellation fee. If the other party accepts, the cancellation fee will be sent to the seller and the partial refund will be sent to the buyer. Example below:
If you need any further assistance, don't hesitate to contact our friendly support team at firstname.lastname@example.org