I've come back to check on this game after an entire +0.1 to the version and see that selling milk in industrial quantities is no longer very profitable (4,800/day/person seems to be the max I can get). So what's the best way to earn copious amounts of flames now?
Farming demon tiles and enslaving them seems to be a fairly profitable method.
Explore the dark alley tiles (demon tiles) and beat the succubus/incubus that spawn there, take their weapons (to sell them) and enslave the demon, then sell them at the slave market. If you have the Bondage Applier fetish, you can enchant any piece of clothing with enslavement, for zero cost, then sell the demon and the weapons.
You can enchant TF potions with + fluid storage and sell it for a tidy profit.
You need 3 water spells, to reduce the cost of fluid enchantments to 0 essences, and it then becomes a click fest of buying TF potions from Ralph, enchanting them with + fluid storage and selling it back to Ralph.
You can save enchantments, so buy up all the cheap TF potions you can, add as many major fluid (cum/milk) storage enchantments as you can (100 is the limit) and save the recipe. Then buy all the potions that Ralph has available, for that particular TF potion, load up that recipe and click the create button until you run out of potions for that particular racial TF potion that you have.