There's many ways to help with this, here's what I do at Clarity
1. Build an email relationship with your supply. They should have direct access to you, and you should build an ongoing drip campaign that teaches them how to improve their experience (and make more money) on Clarity.
2. For members, you should define community guidelines (Do's & Dont's) and ensure they all review / agree to them - and enforce them proactively.
3. Create a discussion forum (we use LinkedIn group) but you could use VanillaForum.com or similar, and invite members/experts to join to ask questions about your marketplace.
4. Feature experts & members that have had success or are exhibiting the types of behaviours you want from other members. We do that here: http://clarity.fm/customers
Those are the big things ... smaller items would be the copyrighting, design and data you show on profiles to help increase the right behaviour (ex: Search with filter by Last Active, tells experts to be active).
Hope that helps.
There's no magic bullet, but here are some bullet points for what has helped us over the years:
1) Be authentic to your product and your company. Share, and repeat that authentic voice wherever you can.
2) Identify patterns in your "high usage" members and encourage all members to do the same.
3) Don't bend to every request of the customer; be confident in what you're delivering, establishing yourself as a thought leader.
4) Try to drive any and all communication with your members through the channels you're targeting. For example, if someone emails you a support question, re-paste it in your community (along with your response) and respond to the requester individually telling them their answer is at the URL.
5) Communicate transparently in terms of what your intentions are with the marketplace and what you're doing to improve it.
It's tough without knowing more about your specific market place. Happy to chat to get more in-depth.
Enabling and Reflecting: helping them "do their thing" their way, and creating iconography that feels "homey". Do users love sharing content? Consider UI/UX approach to share ability of content, usability of sharing buttons. Reflecting includes subtle cues : your choice of vectors, colors, even a demographic appropriate sense of humor in the naming of buttons and links. I read about how even a 404 error page should be included thematically. Remember: you set a culture, but if you get users, they will own it!