As PROM Marketplace is just launched, we deliberately haven't introduced any platform fees yet. Yeah, for now you can trade NFTs at PROM without having to pay anything but the gas fees!
In the future platform fee will be introduced as a share of a sale total amount, deducted from the successful sales. This means that buyers won't face any additional charges except from the amounts stated by sellers.
We'll inform our community in advance when platform fees are going to be introduced.