Quick answer
Assuming you have no offers or pending orders on your listing, you can cancel a listing by selecting Edit, then End Listing next to the item on your Listings Page.
There is no fee associated with ending your Reverb listing. We only charge you a selling fee if and when your item actually sells.
If you're ending a listing to sell it to a buyer who found your gear on Reverb, please select, Reverb found me a buyer -- Charge me 5% when ending.