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 3 dots in top left corner of the mobile app and selecting Listings & Buyers or in the web app, click the Listings & Buyers button on the left side of the screen
Tap the listing/buyer you want to archive
Tap Edit in the top right corner
Tap the Archive button at the bottom
Note: If you ever need to unarchive a listing, you can do so by following the steps here.