Since I've successfully launched a marketplace and if I were to do it all over again, I would take a very methodical approach. My recommendation is the following:

1. Create an Opportunity Assessment. In other words, get your thoughts on paper about the opportunity. Start high-level doing secondary research. Determine your Total Available Market (TAM), your Serviceable Available Market (SAM) and your Service Obtainable Market (SOM). Make some assertions about pain points. Create customer personas.
2. Get out of the building. Talk to 10 prospects about their pain points and validate your assumptions about your customer persona.
3. Build a non-scalable MVP. Building a platform is a lot of work, money and effort. Before you go off to the races, see if you can service 10 customers manually.
4. Service those clients and account for your lessons learned. What did you learn in that process? Were you able to determine where the friction in your marketplace is (supply or demand side)? If you were to build a platform, what kind of prototype would you need to build? What features do your customers need to see?
5. Rinse and repeat with 20 additional customers.
6. Rinse and repeat with an additional 20 customers (total of 50).

If you go this route, I can guarantee you, you'll come out really know your customer and how your product/service needs to be developed. I hope this was helpful.