You can cancel an auction or sale at any time, but there are impacts in doing this.
To end a public auction, click on "Complete and End Listing" under Listing Settings on your Listing page.
If your reserve price has been met, the buyer who placed the highest bid will be declared the winner.
If your reserve price hasn't been met, the auction will end without a winner.
To end a private sale, click on the "End Sale Early" link on the Sale Details page. The sale will end without a winner.