1) Dock in Station.
2) Bring up Galaxy Map.
3) Check a nearby system:
a) If the system is an Industrial System - they will buy metal
b) If the system is a refinery system - they will buy minerals
c) If it is an Extraction system - they will buy Mineral Extractors and Explosives.
4) Now while at the Galactic Map, check the system you are in:
a) An Industrial System sells machinery cheap.
b) A refinery sells metal cheap
c) An Extraction site sells minerals cheap
5) The connection between points 4 and 5 should now be completely obvious!
a) Buy Minerals from and Extraction system and sell in a Refinery System
b) Buy Machinary in a Industrial System and Sell in a High Tech System.
c) Buy Metal at a Refinary system and sell to an Industrial System.
No need for a galactic wide price list - this is far, far easier!!! You instantly know what the system is going to buy before even visiting. And you instantly know where to buy a product cheap without even visiting a system.
This is why the Galactic Trade Map only tells you info about Categories and not individual products. The individual product is largely irrelevant for basic trading.
There are some outliers to this, where you can sell in a more advanced way. But there really isn't a need. Following the guide above you can make 3.5k profit per run in a Sidewinder within about 1hr or play.