Multisided Platforms have the chicken and egg problem, to build them, in the beginning, you need to do things that don't scale:
1- Go to the most difficult side (users) to adopt the marketplace and make them a demo. ex: buyers
2 - Ask them what they need to buy
3 - Then go look for the sellers of what they need and onboard them and call the buyers...
4 - Do this before you start too much coding...you will need to do the old way without tech or just with basic MVP, to understand if they need your product
5 - You validate the idea while you speak directly with all sides, and can improve the product with their feedback.