In the instructions it says that you can select the resource to trade away in return for the percentage amounts of other resources. Currently, I am always trading away food, even if I select another resource and change the percentages for that one to 0%
Example:
I have excess wood that I want to trade away, so I click on wood in the trading window. Then I set the percentages like this: Food 5%, Wood 0%, Stone 40%, Metal 55%. I close the trading window and wait for most traders to finish their current journey. I still find them to be carrying lots of food (more than 5% of traded goods) and never trading wood at all. If I open the trading window, food is selected as the trading resource again.