Oh good lord. Three pages and NOT ONE OF YOU mentions system economies?
*sigh*
Alright. Economy 101 crash course.
In the galaxy map, you can find the option to filter systems by economy type. Extraction, Refinery, Industry, Agricultural, Terriforming, High Tech, Military, and Tourism.
Each system produces a certain type of good. You can look at the information in the commodities menu in any marketplace. Clicking on an item will display more information on that good on the right hand side of the screen. The most important things you're looking for are the following.
Economies that Produce this Good.
And Economies that Consume this Good.
By knowing which Economies produce and consume say... Gold. You now know which systems to visit to purchase this good. Generally Refineries and Extraction Stations. These systems will not only produce Gold but Gold below Average market value.
Buy low.
Gold is said to be consumed by Industrial and High Tech economies. Therefore by taking it to one of those systems, the station will pay ABOVE market price allowing you to sell at a profit. You try to sell it anywhere else, you'll be lucky to break even but likely will lose money.
Sell high.
FAIR WARNING: You can generally expect to make a profit but there will be the occasional situation where for some reason, the good isn't in demand despite being the economy that consumes it. This is rare so you can safely trade at a profit but if it does happen, the game isn't broken. Just note not to try and trade that item there again.
Economy 102.
Not only will the good tell you what types of economies produce that good, it may also tell you WHERE THAT EXACT STATION IS GETTING IT'S SUPPLY. In the case of consumption, it will tell you the exact system the good is getting Imported from. In the case of supply. It will tell you the exact systems the good is getting Exported to. You can see these colorful trails on the galaxy map once you've visited a system and accessed it's marketplace.
Just follow the trail and the money.
Hybrid stations and multi-economy systems:
Some stations (noticeably High Tech stations) have two economies. In many cases one may be consuming the goods that one economy produces so don't expect to find what you're looking for. Some systems have multiple stations and each station may be a different economy. (My home system is an Agricultural system but has two stations. One is Agricultural and the other is Industrial.)
You can always tell what economy a station is on the system map by selecting it.
Economy 103:
Certain goods are only produced in certain areas. This can be affected by which minor faction is in control of the station and the type of goverment. It may be determined by whether the station in question is in space or planetside. It may be that the good is only produced in a certain system or a certain radius around that system. Again all this information can be found on the right hand side of the screen when you click a good. BE WARY OF ACCEPTING MISSIONS THAT DEMAND THIS KIND OF GOOD.
Using these rules I've been able to trade at a profit without the use of exploits or third party tools. Sure it's not millions upon millions of credits but profit none the less.