Here at Segment we put a lot of thought into how to grow a marketplace. Segment is one API that collects customer tracking data and sends to over 160+ tools (analytics, email, marketing automation, CRMs, even databases like Redshift).
At the very beginning, we built out the integrations from our API to the end tools (since no one cared about us). Since at that time there was no "marketplace", we had to heavily subsidize one side (integrations) to appeal to the other side (customers).
We've grown to the point where now there are many customers, when approached by tool X, will ask them to join Segment in order to win their business. Moreover, our integration partners now are a growing source of leads/business (better have us "own"/maintain the libraries on the collection side, plus its easy access to all of our customer base).
We modeled out the "growth loop" for two sided market places here:
Hope that helps! Happy to answer any further questions, too.