Each vehicle has a 30-day listing period. Vehicles can only be displayed as a match in a search when they have a positive number in the Days Left field. Out-dated vehicles, while not deleted from the database, can't be seen by users searching for vehicles. In other words, a truck will not be a displayed in anyone's search results even if it matches a query.
Check the right-most box, Select, of the vehicles to be changed.
Click the Extend link at the top of the page under "Actions" to extend online availability 30 days.