It is not possible to cancel a sale that has already happened. They want to protect your right to use your own ticket but also the right of the buyer to make a safe purchase. So to avoid uncertainty: once you have posted the listing and the buyer accepts the price and terms and conditions by purchasing the ticket, you cannot withdraw your offer anymore.
If you offer tickets on other platforms, you actively accept the risk of selling the tickets twice. They therefore strongly recommend not offering tickets on other channels while they are offered on Ticombo.