How can I implement reverse bidding into Wordpress?

I currently am developing a client website which involves a few components, however one of the biggest areas is that my client wants to create a marketplace for expertise (similar to Elance, ODesk ,etc.)

I wanted to ask if there are any simple ways to get this done (i.e. an addon plugin to modify a shopping cart like Marketpress) or if this is something that would need to be built from scratch.

Thanks very much in advance,