Once a listing/buyer is sold or has fallen through and you no longer have data you need to tag to them, you can archive them. Once you do so, they no longer appear in the options of listings/buyers to tag for mileage, expenses, and income.
To archive a listing or buyer:
Tap the 3 dots in the top left corner of the mobile app and select ‘Listings & Buyers’ or click on the Listings & Buyers page on the left hand side in the web app
Select the listing/buyer you want to archive
Select “Edit” in the top right corner
Select “Archive” at the bottom
Note: If you ever need to unarchive a listing, you can do so by following the steps here.