It depends

When we started Wooshii we did very little to build supply. It was all about demand.

Our theory was "Build enough demand and finding supply is way easier."
It meant we didn't have to engage our community in other, (most likely distracting ways)

You can always deal with demand in an unscalable way early on. FInd some trusted suppliers to pick up the majority of the demand.

It worked out for us as we are service led. The last thing we wanted was a massive video community and no work for them

Another way to have done that might have been to create a sort of Single Player equivalent. Choose the side that has the most pain and help ease it with some other tool.

Of course with product or inventory type marketplaces supply may be the way forward - building an inventory of scarce collectables ( as eBay did) then allows you to start itching your product to the demand side

Number of posts

http://thenextweb.com/entrepreneur/2013/12/08/8-strategic-hacks-building-marketplace-liquidity/

http://techcrunch.com/2012/08/19/how-to-structure-a-marketplace/

http://karlinvc.com/blog/how-to-build-a-defensible-two-sided-marketplace/