I would definitely use this selling, probably for buying too. Selling a car is a pain in the ass, and probably the three biggest pain points are: 1) fielding phone calls, 2) setting time aside for appointments, and 3) dealing with people that may or may not be serious about paying the asking price. This addresses all three.
Let's build this for selling/buying homes. I have sold and bought a home recently and I did so without the use of a listing or buying agent. It can be done, easily and you generally save at least 3% on the purchase and sell side. Therefore, eliminating it would equate to a savings of around 6%. You could charge 1% of the sale. The average home price in 2010 was $227k. One percent isn't too shabby. Plus, think of the referral fees you could grab from inspectors, attorneys etc. Who wants to build this with me?