It is possible, though initially quite risky (as you'll need to run with very thin margins of error to make a reasonable profit in a reasonable time) to make money off of trade. The trick is to make sure you know where all the big and reliable consumers and producers are. Precise quantities of supply and demand may vary, but market conditions rarely do, outside of special events and food shortages.
The biggest obstacle to a prospective trader are all these damn pirates running around. It's even worse if you're playing with mods, because then you've probably got Underworld, and thus you probably have the Cabal flying around, too. Not to mention that the pirates and Pathers will have a nastier and more varied toolkit. The resources you need to invest in to fend them off reliably will seriously cut into your profit margins. Minimizing the escort ships you need by making your efficient freighters and tankers double as converted carriers might prove worthwhile, since it will provide some reasonable combat power without increasing the amount of credits you spend on supplies and fuel.
Procurement contracts can be quite profitable, but access to them is fairly random.
Between needing to keep track of where the suppliers and consumers are, how quickly it takes to move between them, and the knowledge and skill needed to cost-efficiently fend off various groups of pirates and terrorists, you already need to be quite skilled and knowledgeable about the game to pull it off, at which point you're still not pulling in any more money than you would make off of bounty hunting, considerably less than you would from exploration, and odds are you're having less fun along the way.
Trading is thus mainly opportunistic, something that mostly happens when you notice that the present situation is set up conveniently. For instance, if you're in one of the big Luddic systems (for whatever reason) and a food shortage occurs or you happen to be heading in the direction of Chicomoztoc, you might as well pick up a big quantity of food or domestic goods and sell it there. Or, more likely, if you happen to be at or very close to a market with a given commodity in surplus and you spot a procurement contract for that commodity.
It's also a decent source of income if you happen to be doing a pirate run. Of course, then you don't actually have to pay for the commodities, though finding an appropriate target can itself be an annoying process.