Profits becoming harder to make later in the game is not related to a high trading skill, though, I feel.
It's mostly that the general economy goes to **** after a while from the constant wars and aggressions and lack of a deeper structure in relations to diplomacy between kingdoms, and the AI not defending its territory more, and caravans making a mess in many areas demands in ressources, etc.
It's much more complicated than just naming a problem and fixing it, it will need a lot of work.
We'll need some patience.
I am aware that whole economy breaks as time passes. What I'm talking about is that goods prices are starting to flatten when I'm raising trade skill. It's whole different issue.
For example grain price at jelkala is at 8 with 300+ in stock at start with trading at 5 then when I raised my trading to 50 same city, same stock, price is going up and is at 12. City situation is the same prosperity on same lvl. Villages never raided, city never under siege.
What I try to say is that my buying prices go up even in places that are not changing while selling prices drop even if there is demand.