Check out ShareTribe’s open source code for their marketplaces:
If you know how to code, that may be a good place to start, especially if you are trying to create a P2P (peer-to-peer) marketplace where people can buy and sell records from others (for example) as opposed to a few companies offering services to a lot of potential customers.
If you are in the latter situation, you can launch an MVP using a WordPress theme built for marketplaces. That would be cheapest.
In either case, the most expensive aspect of this venture will most likely be attracting both types of users.