You can your transaction at any time before the buyer has agreed to the terms. To cancel, you need to log into Escrow.com and click the cancel button on the transaction details screen.
If the buyer has already accepted to the terms of the transaction, you can still cancel by emailing firstname.lastname@example.org to request a transaction cancellation.
If the buyer has already paid escrow.com then both you and the buyer will need to email Escrow.com support to request the cancellation. If both parties do not agree to the cancel the transaction, then Escrow.com can not cancel. This only applies if the buyer has already been paid. Otherwise, either party can cancel at any point.