Every listing stays online for the number of days on the Marketplace Pass that unlocked it - 30, 60 or 90 - counted from the moment it was posted. A Marketplace Bump moves it back to the top and adds more days on top of the days it still has left. When the time runs out the listing is not deleted: it moves to the board archive and stays in "My listings", where one more pass brings it back online with a fresh countdown. A listing you mark as Sold is archived 3 days later.