It is indeed profitable (and has been successfully practiced and is successfully practiced by me).
The normal food price with a stockpile x, with 2499 < x < 5000, is between 3 and 4 RD (without Marston Fort effect, ie Marston Fort not occupied).
Food price is increased by 1, if the food stockpile x is below 2500. The Food price is increased by another 1, if the food stockpile is exactly zero.
Food price is reduced by 1, if the food stockpile is 5000 or higher.
Now during the new food price calculation first the new prices are calculated, taking into account the current food stockpile, afterwards the amount of food at the market is adjusted.
The difference between buying price and selling price is normally 1, ie. selling price is 1 lower than buying price.
Lets assume the food stockpile is zero, then the lowest possible price (without Marston Fort effect and without any other effect ie. from missing fusion power plants) is 5 and thus the selling price is 4.
Lets assume the food stockpile is 5000 or above, then the highest possible price (without Marston Fort effect etc.) is 3.
The win margin in the worst possible case is thus 1 per ration, with about 5000 rations to be bought and sold that would be a profit of 5000 RD. And a trip to the Redhaven market (with a non-limping mc) just takes 6 minutes (if one leaves the home anyway for whatever reason), so it takes alltogether just 12 minutes for a buying and selling cycle.
The lowest possible (buying) price for a food stockpile above 5000 is 2 (per ration). The highest possible price for a food stockpile (being exactly zero) is 6 (and thus the selling price is 5) - without any additional effects in place and normal prosperity/control levels. The highest win margin per ration is thus 3 or 15000 RD for 5000 rations.
So you will make a profit every two days (without any other effects) between 5000 and 15000 RD or between 2500 and 7500 RD per day, for an ingame time investment of just 6 minutes each day (assuming that there is no other reason to go to the market and you are leaving your home anyway).
Now the price is raised by 3, if the Marston Fort is occupied, but if the price adjustment (which happens at midnight) has already happened for that day, the price adjustment is postponed until the next regular price adjustment (and only then the food price will get adjusted with the "Marston effect", if the Marston Fort has not been cleared before that price adjustment is calculated). So taking advantage of going late to bed (past midnight) and not clearing a bandit band from the Marston Fort immediatly (but only clearing it partly, when it gets occupied, and sparing the last miserable 4 bandits for the next day), can raise the profit by 3 per ration (if your next action is a sell action and not a buy action), so assuming about 5000 rations are traded, that would mean an additional 15000 RD (above the base revenue).
And the chance that the Marston Fort gets occupied is not that low, of course you also might want to not always directly clear the Stokke Fort, if it gets occupied (but only clear it in parts)¹, since it seems (not verified by a look into the code) that the chance that the Marston Fort gets occupied is higher, if the Stokke fort is already occupied (otherwise it is more likely that the Stokke fort gets occupied and not the Marston Fort).
And you just need a food silo and enough cash at hand to start the "food price manipulation". And the food silo is rather cheap - you need the food silo, so that you are able to not let some rations rot at midnight (while you temporarily have the redhaven food market stockpile for safekeeping ;-) ). So the investment hurdle is rather low and the investments will amortize nearly immediatly and it is also worth to take a loan to be able to start the food market price manipulation early.
But I will leave it at that, as otherwise GD-Studios might come to think, that such money gain from the food market should either have some negative consequences (like especially the poorer folks getting angry at the mc/pc or the mc/pc losing influence²) or trying to prevent the money gain (eventhough that might be harder to achieve).
ps. Of course you might occassionally not boost the market stockpile via selling directly to 5000 or more rations, but then you will still know the very next day, if in case you buy at that price you will be afterwards able to sell with a guaranteed profit (or not). And you will always be able to buy that day and sell the next day with a profit, if you did not already sell that day (between midgnight and concluding your day) and the Marston Fort gets occupied during the conclude day phase.
pps. In my current game I did make more money from the food market (via buying and selling) than I would have gained from three won poker games at dockgrave each day always with the maximum possible winnings and that without really selling and buying at the food market every single day (but sometimes forgetting to buy or sell or deliberatly waiting and not always making sure, that the food market had 5000 or more rations are I did sell rations). Of course that is partly also due to the fact, that the Marston Fort in my game did get occupied quiet often.
¹) Another reason is, that some global events can only happen, if the Stokke Fort is occupied (and I like the control rating to not get above 30 for long and preferable keep it between 25 and 29), like the stokke band raiding an ammo depot....
²) But then again, what does a mc/pc, who might also purchase from Shady Dave or extort others, care about a little influence loss.
