This article describes how to remove your ad.

To edit or delete or renew your ad, log in to Yahoo! and then select the My Autos Center tab on Yahoo! Autos. Your listing appears in the "Selling My Ads" section. If this part of your My Auto Center page is not expanded click the "View" button in the My Ads area. From this view you can see the number of views on your vehicle, the status of your ad, and you can edit or remove the ad from this page.

Selecting the "Edit" link takes you to an editing ad details page. Make your desired changes on that page. You can then to see these changes on a preview page. From this preview page you can add photos and make other edits.