Wha??? Sounds like this whole verification nonsense in nothing more than racketeering against small businesses if that figure is true??? Any reference so we can check out?
And would you mind sharing what kinds of costs are involved for a marketplace? I can only think of hosting costs since I don't have much exp in making websites.
It depends on how you build the payments process, but if you (for example) want to commission trades and make everything smooth for users, you need to become a payment processor and that's 500K. Everything else comes on top. There's also security certification etc etc.
CC issuers specifically deny third-party intervention in the sales process. The entity charging must be a registered payment processor, and the other party is the buyer. So the marketplace has no place in that unless registered.
There are some marketplace payment solutions but each has their pros and cons. For the one I worked on, the only solution found without enormously expensive fees was to "force" the sellers to register as merchants with the payment provider via the forms inside the marketplace offered by the payment processor. The marketplace never took off though.
The issue is that you cannot tap directly in the money flow for a comission. That's not allowed. For intangible goods there is also the possibility of late fee-charging, such as later fees charge on the seller's card. But you assume a risk there. If there are any previous costs, such as domain registrations supported for example (not the case I guess), worker salaries, servers, whatever else, you will be at a loss each time a client does sell their stuff but does not pay to you in the end. So... there are a lot of unknowns.
Marketplace building is not for the faint-hearted, nor for the low budget ones.
Edit: for more precise insight you need to talk directly to cc processors, this is just side info as I'm not in the industry, but based on hurdles we have faced.