VP Product and Engineering @ Rev. Previously venture investor and marketing guru in cleantech sector. 6 years as strategy consultant. Love helping entrepreneurs, especially MIT alumni.
From a lean startup approach, try to create the side of the marketplace with which you have the most questions or doubts.
Tom Eisenmann at Harvard Business School has written a few papers on this subject. Try this one for starters. http://hbr.org/2006/10/strategies-for-two-sided-markets/ar/1
We've done a fair amount of this at Rev, building both supply and demand first for remote services.