Marketplaces have a uniquely defensible architecture and business model. Each user adds value to the whole, and long-lasting network effects take hold.
The marketplace architecture is well-suited for understanding and building crypto-economic systems. As I've been studying the topic, I found a 2012 post by Bill Gurley to be instructive. It's titled All Markets Are Not Created Equal: 10 Factors To Consider When Evaluating Digital Marketplaces. Check it out.