Well, this is rather convenient. Do you want do buy or to sell crypt? Actually, there's no difference the system is the same, anyway. Whoever wants to buy or sell the asset, places a bet (bid or offer) into the system. I saw Coinplace announcing hidden dashboards for bigger pool deals but here we're talking about the open part.
So, when there is a match between the two sides, i.e. buyer and seller, the deal is done. Technically, the one who has crypt on his/her hands, places it in a custody (escrow). And the other side transfers fiat. When fiat is received, the "crypt" side unlocks the escrow.