The marketplace will allow companies to post their goods, services and stores in the marketplace, and the buyers will be able to negotiate prices in real time, 1:1, in privacy, 24/7. It will be automated for sellers.
I would go for a Phoenix framework (backend) / ELM (frontend) based platform.
The use of these two functional languages will allow you to support high user concurrency and to serve real-time user experience.
Feel free to reach me if you want to talk about it.