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 this page in the "Selling" section of your Reverb account:
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 in order to sell it to a buyer on Reverb, we'd very much appreciate it if you'd select, "I found my buyer on Reverb -- Charge me 3.5%" when ending.
You can follow the same steps to end a listing on Reverb LP, as well.