“Buy Now” listings can be cancelled at anytime up until the sale has taken place.
Once a sale has taken place it cannot be cancelled or reversed.
How to cancel your listing:
Click onto the NFTs page
Click the “Cancel listing” button
Confirm you're wanting to cancel your listing by clicking “Cancel listing”
Your wallet will prompt you to “confirm” a transaction to complete the process.
You will see a confirmation screen to confirm your listing has been cancelled.