I have worked at a large marketplace company for over 7 years. The general answer to this is to find service providers (or "experts" in clarity speak) first, and then go after the buyers.

1. Getting providers is cheaper than finding buyers
2. Buyers won't come to your site without an "inventory" of providers
3. Providers create SEO karma
4. You will figure out the issues with your idea or service as soon as you start talking to providers.

Of course, the specific answer depends on your business idea.