You know what would be great... A platform exclusive for Marketplace experts ;)

I would answer this in short by saying you don't attract them simultaneously.

To elaborate;

The Chicken and Egg problem is solved (historically) by attracting and retaining the top providers of whatever service first. This, in turn, creates the - "have" to be there or at least submit proposals there, or risk paying more, getting less etc... - notion in the customer's brain.

As for Clarity specifically, watch this video of Dan Martell (founder of Clarity.fm) telling the story of how they did it:
https://www.youtube.com/watch?v=wKQ2nw3UWPk

I have helped launch a few service-to-customer platforms in the last few years. So my related answers and advice are all outlined here:
http://chickenandegg.io/faq/

I'd suggest finding a hook for the experts/service providers first and foremost. Boiling your platform down to a niche is best if you're strapped for cash. And further pinpointing a localized or network-oriented market is a great strategy. Quora, for example, utilized their founders' relationships with VC's in Silicon Valley to gain their influencer early adopters. Thumbtack.com used a creative strategy of creating a tool for their service providers to use (for free) to help them with their craigslist ad posting needs. This added the stickiness they needed to keep those first service providers while they marketed to customers.

Let me know what specific answers I can help you with. I'm better with specifics.

Alex