ED has supply missions, which are analogous to delivery contracts in the present. But in the real world spot markets also exist for a variety of commodities. The settlement time for most spot markets ranges from minutes to days, so the whole idea is that when you make a transaction you have goods parked somewhere ready to deliver. You don't generally stroll up to a bulk shipping terminal unannounced with a fully laden cargo ship, but in principle one could if one had a strong stomach for paperwork.Nope. If we're talking about trading physical assets on a large scale, that is done with contracts. It's not like someone says "I want to buy N amount of things" and you show up with a cargo ship and sell them stuff. You have a pre agreed deal where you deliver N amount of stuff and they pay you X money. The ED market doesn't work like that, so you can't compare it with real world bulk trading. And the markets that work like the market in ED do not exhibit to behavior.
Now obviously one way ED world commodity trading differs from ours is that ED abstracts away the transportation and logistics and time delays. If you have a tanker of crude oil, you have to go to petroleum terminal to unload, whereas if you have a load of bulk grain you probably need to find a grain elevator with available space. In either case, once you've done that, if you don't already have a buyer you could go to the corresponding mercantile exchange to execute an immediate sale on the spot market, or arrange a contract for future delivery. In the ED world the landing pad also acts as a universal cargo terminal, and there would be an army of brokers behind the scenes of that "simple" market prices display. What physically happens once the cargo canisters leave your ship is left entirely abstract.