I am currently the CTO of a vibrant online marketplace.

As prior responders said, we started with demand. Once we had people who wanted to pay for our services, it was easier to find suppliers to fulfill the demand.

In addition, we started with a narrow focus, and then expanded. We are a local service marketplace, so that meant starting with one geography, then expanding to a second, and a third. After we had figured out the formula in three geographies, understood the dynamics of the business thoroughly, then we expanded nationwide.

As we roll out new features and services, we repeat this same pattern again and again: start in one geography, figure out the feature functionality/business dynamics/etc. then expand.

Coming back to the demand point for a moment, in every business, marketplace or not, demand is the hard problem. The fact that you're an online marketplace is irrelevant. So, to be a successful business, say focused on finding repeatable, scalable and affordable ways to generate demand and you'll be on the right path.