Go to an Imperial system. In most cases there will be a mission to source two units of imperial slaves for around 45 000 Cr.
Notice that the average price of Imperial slaves are around 10 500 Cr/unit. I hope you do not need me to take a screenshot to prove this.
Now, notice the sourcing missions that most starports offer you. Note that the higher paying large ones usually are locked by reputation. You will notice, that they offer quite a good profit, indeed, a lot better than what the commodity market offers you.
For ordinary trade, I suggest Superconductors or rare metals if you want to earn a lot per tonne.
yes I know how to trade, I already made 6 million on SC's and Indium before the nerf, those items are one third their original value now.
I know that you believe that everyone else in this thread is wrong and you are right, funny how there's only one of you.