You have to pay a lot of attention to shortages and surpluses. In my game, for example, Chalcedon is consistently short of Marines, meaning they'll pay 300+ for them, and there's almost always at least one system within ~7 light years selling them for ~130 a pop. Even if you pay the tariff on the purchasing end (you shouldn't ever need to pay at Chalcedon, the Pathers have almost no fleet strength) you'll be making over 100 profit per unit per trip, and you can do it a couple of times a month at least. Similar, the pirate Spaceworks - I forget the name, it's in the same area - is always hungry for both kinds of ore, so one profitable triangle tends to be the pirate station in Sindri (buy ores and marines) to the pirate spaceworks (sell ores, buy marines and drugs) to Chalcedon (sell marines and drugs, you might be able to buy organs or might not).
The main trick I think is getting up to a size where you can make those trips profitable. Since trade depends on taking advantage of shortages and surpluses, you need to have several hundred or ideally a couple of thousand free cargo capacity at any given time, so if you end up somewhere that has a discount you can snap the discounted item up in bulk. Landing on a pirate station and unloading 1000 units of drugs or heavy weapons while a shortage is going on could net you hundreds of thousands of credits in one run.
I trade with Pirates and Luddites a lot - they don't patrol much if at all and insist you have your transponder off, so you can use the Black Market all the time which is 60% extra raw profit. Trading with major factions profitably will most of the time rely on either taking advantage of very lopsided shortage deals, or sneaking past patrols while Running Dark, or just dumping all your illegal stuff on the Black Market with your transponder on and then whistling innocently when the patrol shows up to scan you. You can mix BM and open transactions in order to lower your suspicion, which can sometimes be worthwhile; if you spend or earn roughly equivalent amounts in both, patrols usually won't come screaming up your ass right away.
Anyway, point being; it's absolutely practical to make money trading. I do it very inefficiently, in a gas-guzzling fleet that includes capital ships and several dedicated combat vessels, and I still clear two or three hundred thousand a month in trade profit usually; focus on fast freighters and you could probably break half a million per month. If you're careful about smuggling to major powers it doesn't even hurt your rep much.
