the ingame UI is totally wrong and doesn't provide you with enough information to figure out what commodity is best sold in your current cluster of systems, without having to actually go to everyone yourself.
Well, it does enough to tell you not to take food to an agriculture system, doesn't it?
How about not taking stuff from one High Tech place to another? That's in the UI too, isn't it?
So, you can sort systems by what type of economy they are. You can sort them by population size. You can see what the current station you're at has in high supply and how much they want for it.
The trouble you're having is you need to make an educated guess at which to choose trading, and then where to take it. You're not educated enough to be getting it right if you're not making 300cr-900cr on simple runs - I am. I'm using the same tools as you and I'm trading, for example, progenitor cells, performance enhancers, land enrichment systems...et al, from high tech systems to agricultural systems and I am always making a nice profit.
Then I take something back to the high tech places.
My latest run nets me in excess of 500,000cr profit between three stations, that have several jumps in between.
I use the UI and nothing else - but
YOU DO HAVE TO VISIT PLACES AND SEE FOR YOURSELF!!
The only thing I agree with, is that the places you have visited need to be logged for easy reference, so you don't have to rely on your memory (mine is aged) or have to find a pen!