The whole thing needs legal, insurance and escrow. Nobody in their right mind would want to host it for free if it actually handled the money. Could go anonymous and crypto but really that’s just asking for it.

Hooking up buyers and sellers and having them sort out prices, taxes and shipping would prob be the only viable way.

At some point, someone is going to do enough business to get a good rep and make a large sale and screw someone out of money.