The marketplace operator would have to first allow the marketplace user seller privileges before they would be able to sell. There could be some rules set in place such as the user must have a certain amount of positive feedback before they would be able to sell.
But if no one is allowed to sell and can only buy in the beginning, who will be creating the sell orders? Will you have a whitelisted team of sellers that you personally select and give them selling privileges as soon as they register? Seems a bit unfair to be honest.
Both Ebay and Amazon never used an escrow service that made sellers wait to receive payment for the items they sold and shipped. Not using an escrow seemed to have worked out well for them.
Yes, but they have buyer protection. I will get my money back if you ship bogus items and provide services that don't work as advertised.