Maintain a searchable listing inventory of where landlords are choosing to with-hold rentable units. This component would be a crowd based site to bring vacant units to market for rent. You could co-relate data streams such as "power-on, water-on" or other city services to make a reasonable assumption regarding if the unit is occupied or not and if it is NOT occupied provide contact info to the building owner (is this public info?) to put pressure on them to rent out the unit. The more rental units available the more prices could stabilize / drop.