You can remove your Mart Item by changing it to Closed status at any time.  Navigate to the Open Items tab on your My Mart page, then click the Close button next to your item's listing.

There is also a Remove Item link on the actual ad listing, but this is only visible to the member who listed the ad and only when they are logged in.  Clicking this link will work in the same way as clicking the Close button on your My Mart page.