Abstract: Methods and systems for facilitating the sale, lease, rental, or other monetization of unsold space inventory (e.g., seats, berths, cabins, rooms, etc.) to customers or passengers as guaranteed, empty adjacent space, after the passenger has already purchased a reservation and check-in has commenced. The empty adjacent space (EAS) may be sold as an upgrade when a guest checks-in at an event (e.g., an airline flight, theater performance, train trip, bus trip, hotel stay, day at the beach, parking space lease, etc.) or some time thereafter.