There are two ways to make money as a pure market player: connecting buyers and sellers who wouldn't trade directly (and taking your cut) aka arbitrage, or getting paid to take on risk. Maybe the trader notices that people on the far side of the island are hungry but can't get a big mac without a long walk, so he buys as many as he can carry, walks over to the other side of the island, and sells them for a bit more than they'd cost on the dockside - that's the first kind of trading. Maybe a clamshell farmer wants to make sure they can afford enough big macs over the next year. They might agree to sell their next year's clamshell harvest at a fixed price (that's lower than the average), or buy their big macs for the next year at a fixed price (that's higher than the average), or both; the farmer loses value on average, but they offloaded their risk, while the trader makes money on average but has to carefully manage their risk.
The third thing is to actually take an active position in the market, if you can predict what's coming. E.g. if you realise the clamshell harvest will be big this year, maybe you sell a bunch of clamshells short. That should send a useful signal to all the farmers, and it means you make money if you predicted right.
