I've built two £1m+ businesses (0800handyman and KeepMeBooked) from start-up to sale.

Provide something of value to one side or other of the marketplace first. For example:

- A mileage and revenue tracking tool for taxi drivers (Hailo)
- A place for designers to publish their portfolios (Dribbble)
- A reservation system (HostelWorld)

That way, you can get suppliers on board without them expecting any revenue from you. You are providing them with a useful tool. Once you've got enough suppliers using your tool, you can then go out and generate demand for them and turn your network of tool users into a marketplace.

More here:

http://pando.com/2012/11/20/liquidity-hacking-how-to-build-a-two-sided-marketplace/