I have worked on a B2B marketplace for technology and advised for a few B2B vertical marketplaces as well. This was for different clients in US, Asia and Middle East.
You will need:
1. ability to attract integrators and sellers
2. Easy onboarding
3. Low listing fees
4. Great marketing team
5. A compelling reason to adopt your marketplace instead of alternative options and "current ways of doing business".

If you have these, your traffic will start growing, which would lead to signups and so on.