I don't remember exactly, but I believe that while the traders split the items they sell equally, they buy the cheapest items you are selling first and move up to the next level once you have maxed the items you can sell at that price.
I watched that I always sold the high priced items first and then released the lower priced items as the year progressed. This takes a good deal of micromanagement, but you get to maximize your income and if there is any shortfall, at least you've sold from the top down.
The hard thing is not getting distracted when January rolls around and you don't reset your trade mix before the first trader does any trading.
When possible, I try to sell only the high priced items, one per each customer city. This means ensuring that the items you sell are the highest priced items per city and avoiding conflicts with other cities.
Generally speaking though, it is best to sell only processed goods, unless you can sell tons of raw materials (40+). This is where you can make a great deal of money. Of course, don't short your own city, or you will suffer for that profit.