In "the real world", when I buy a stock through Scottrade or whatever, I can set up a rule that if the price of the stock goes up to some level, or goes down to some level, the system will immediately list it for sale.
I want something similar for the black market. For example, any time my water supply reaches at least 90% capacity, I want to sell it at any price.
Meanwhile, I might want to set another rule like any time my food supply hits 80% capacity, or even some amount like 150,000 units, and the price of food exports is at least 100 per 250 units, I want to sell.