The best thing for you to do is to copy Clarity.fm

THis site is the perfect example for you.
It's number one key mode is the Answers section - here the buyers and sellers can watch, read, test out and ultimately make a decision to buy.
I get people contacting me BECAUSE I write Answers on Clarity.fm so I'm proof this works.

One word of caution, beware having a big imbalance of buyers and sellers. You need experts in the marketplace before you attract buyers, BUT experts won't get involved and won't engage and come back over and over unless people are actually buying. There is a big marketing communications piece which connects your site back to individuals' inboxes.

Here is an example of a failing marketplace http://blurgroup.com/
With a little research you can see there is an evident imbalance in buyers and sellers and very few recently-dated jobs listed. It's clear this will not last.