There isn't any real Demand & Supply, since we don't have a real (Sandbox) Economy.
All a demand i.e. of 0 indicates is that the rubber band for the Demand figures is currently pushed into its maximum limit, which will (temporarily) affect the price.
Thus, all you'll ever get from the Economy is various Commodities fluctuating in price according to Faction States (primary factor) and current position of the rubber band (secondary factor).
There's still alot of places that sell a Commodity and happily purchase (at a profit) unlimited quantities of them (purchased at a lower price elsewhere), without even affecting the indicated Supply figures in the slightest.
Just accept that and utilize that knowledge to maximize profits. That's what the Commodities are mostly made for. There's nothing deeper to it and no actual Economy to be satisfied.