I've found a trade exploit with food. You can make infinite profit with a single stack of food by abusing with the food shortage of cities.
To do this :
Buy a very high quantity of food from a low price place (for exemple fish from the north) like 2000 unities of fish.
Go sell the full stock to a city with a food shortage malus, so with a high price.
Wait 1 or 2 days to wait for the food shortage to disapear and have the city with a bonus food stock.
Now you can buyback your full stock of food with a lower price.
Do it again with a another city, you can come back some days later on the 1st one.
To do this :
Buy a very high quantity of food from a low price place (for exemple fish from the north) like 2000 unities of fish.
Go sell the full stock to a city with a food shortage malus, so with a high price.
Wait 1 or 2 days to wait for the food shortage to disapear and have the city with a bonus food stock.
Now you can buyback your full stock of food with a lower price.
Do it again with a another city, you can come back some days later on the 1st one.
