OP I apologise if this is a bit basic and you knew it all, but just in case :
There is a basic supply demand chain.
An extraction economy digs up minerals.
You can sell minerals to a refinery economy who turn them into metals
Metals are bought by industrial economies who turn them into products
Products are bought by most economies (mining equip to refineries, agri equip to agri, computer clobber to high tech)
Hovering over a commodity in System X, it will actually tell you in the right side panel which local systems it exports too. This is accurate. As to the price...well, I typically take say 10 of 3 things there that might be a good deal...and then remember which one is the money maker.
That is the framework. There is ALOT more to it.
I always set my Galactic Map to 'Show Economy'. I then filter out the systems of the same type. Or only the type I want to fly to.
I then look at the systems and discern the high pop/rich ones. I then look at the system map and try and estimate if the station is one of those annoying 'far away' from the mother star.
I then check the bulletin board to see if there are also any missions that involve me taking stuff to my candidate destinations...or indeed if they need stuff FROM one of my candidate destinations.
It is best to 'work' a system...pick a home...and work 50ly or so around it. You build up intel on the best prices, you build up rep and you learn which are the stoopid 3487293478927497239479273489ls away platforms.
Currently for nostalgia reasons I play out of Lave...where on the 16th I shall press the cause of the alliance (hoping the galactic struggle for the emperor is in some way played out in Lave).
Some economies are mixed. The population of a system dictates its supply/demand turnover.