You can trust or enforce the shipment of goods monetarily if the money only flows when contracts are fulfilled. That's the whole point. People don't get paid unless they send goods. They don't have to send goods until money is provably locked in a form of escrow that can be settled immediately upon the arrival & inspection of the goods. With gold you can either ship it first & take all the risk, or they can ship the goods first & take all the risk, but there's no way to balance the risk. And the added cost of shipping gold doubles the cost of the exchange.