Traders must invest in trade goods, which are an abstract resource costing one bank note (1000 coins) each. Trade goods must be purchased with bank notes, which requires a bank relationship. Nobody is going to entrust their merchandise to a group of vagabonds with a sack of coin. Each trade good requires a wagon and team, making it obvious at a glance the approximate value of any caravan. For simplicity’s sake, the cost of wagon and driver is subsumed into the cost of trade goods.
Arbitrage gained is equal to the number of random encounter checks due to travel braved for each unit of trade goods. If the journey was not perilous, characters other than adventurers would already be moving goods. Upon reaching a destination market, the party gains a return of 50 coins in exchange for each point of arbitrage.
For example, a party with three units of trade goods that faces four random encounter checks accumulates four arbitrage points. This translates to a return of 200 coins on each unit of goods, yielding a total of 3 (trade goods) x 4 (arbitrage points) x 50 (arbitrage return) = 3 x 200 = 600 coins, which must then be divided among the party and is treated the same way as treasure. The way I usually run wilderness travel, each hex takes one day to traverse at standard overland speed, with one random encounter check (1 in 6 chance) per day and one per night. This means that the “cost” of the above example return is 3 bank notes worth of capital (3000 coins) and 6 random encounter checks. Increase either the capital invested or the distance travelled and the return increases proportionally.
There should probably be some limit to the amount of trade a given town or stronghold can absorb, but that can be handled by common sense and ruling. Return can be adjusted for goods that are particularly in demand if desired, though this requires slightly more settlement elaboration on the part of the referee. Perhaps tags per settlement for goods exported and imported would be enough to support this added level of detail. I vaguely recall An Echo, Resounding and Dungeon World (the steading system) to have some related ideas, so perhaps they can be mined for approaches to managing settlement information.