Competition. If companies are making huge margins by simply providing brokering and marketing services (instead of also providing assets like a car or a gallery), more companies will get in the space and drive the cost down.
I think you're underestimating the network effect barrier to entry (or infra cost in ZipCar's case).
Look at what eBay's head start bought them and what they've done to drive up margins since they won their market. A viable competitor would need to have a significantly better product.
