I've coded a, unique I can say, Marketplace plugin for for vB4. While there are lot of sites for Forum Admins, they focus in discussion with a small section for listing plugins.
The current sections of my plugin are:
Multi seller shop where coders/designers can sell their mods/styles. Can work like Codecanyon where money goes to admin and then to sellers or direct to sellers. Comission can be fixed amount or percentage of the sales amount. Both options can be different per usergroup.
Freelancer where clients can post requests for custom work and freelancers can bid on them. Freelancer can set who will pays the sites fees (if any) and also create milestones for payments.
Classifieds for selling licenses, domain name, websites. Again the money and depending on the usergroup permissions can go to admin or directly to seller.
Disputes. Maybe selfish, but I believe that is much better than PayPal resolution center.
Wallet: A separate section holding all Credit/Debits for Users/Freelancers
Some extra points:
Every post in Shop, Freelancer, Classifieds can have it's own thread, depending on usergroup permissions.
I'm thinking to add my HelpDesk mod, but I'm not sure for it.
The problem is that, even if I know that is a good working plugin and (as I know) there are no similar sites, I've doubts to start operating a website.