To cancel a sale as a seller: First comment on the sale page for the transaction and inform your buyer on why you won't be fulfilling the sale. You should then promptly refund your buyer’s full payment and let our support team know when the refund has been issued. Alert our support team by leaving another comment on the sale page and checking the "I need help (alerts Swappa Staff)" box.

See also: How to cancel a listing as a seller, and How to request a cancelation as a buyer.