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.