Unlike the fixed prices of the older Elite games, the demand for certain goods and their prices fluctuate all the time because even in the single player game they are linked to the multiplayer universe, so when players have provided enough goods in demand at one location the price falls and the profit vanishes. It is much harder to trade than it used to be, but more like real life. You can see when you look at a list of goods the systems demanding them, so you can write those down and then see if any of those systems can be profitable.