If the price isn't cheaper than Amazon or eBay, and the site don't have good amount money to advertise their sites, no one will find the marketplaces and didn't even interested to use it.
An open and fee marketplace would be overwhelmed by fake products, sellers and reviews. There would be no safe way of exchanging or returning products.
There's, they can use smart contract.
The buyer need to confirm if he received the right product, then the funds will be automatically transferred to the seller. In case if the seller send a fake product, the funds that still locked can be retrieved to the buyer.