The marketplace creator could act as an administrator, who has the power to ban scammers.

When the marketplace is created, the following message is sent:

market_info/description/seller_fee/buyer_fee

Fees are one-time payments to the administrator to be able to participate in the marketplace. They can also be zero if the administrator so decides.

Administrator sends the following message to ban a user:

ban_account/account_number

Anyone not satistfied with the policies of the administrator can easily create their own marketplace.