This is the model I have been using in my app. The goal is to have your vendors/customers be a function and asset of your service, provided to the free user base.

It's really all about extremely targeted but transparent marketing as well as finding interesting ways to utilize user data. The data and its interpretation is what is really valuable, and that is what the customers are paying for. It cant just be simple advertising though, it has to resolve a shared necessity of both customer and user.