What causes the trade window to be grayed out in the Trading Post?

I started playing Banished today, and I'm having problems with the Trading Post. The last two ships to come to the Trading Post, I have been unable to trade with. The ship currently at my dock has cattle for sale, and I have over 3000 food (corn, peppers, and fish) available to barter.

For some reason, the right hand side of the trade window is grayed out. My desired inventory matches my inventory on hand, so I know its not that. At first, I blamed my workers, but even after adding more workers, I can't get the right side of the trade window to enable.

I have checked the in game help system, as well as the wiki, but I've not been able to find an answer. Any ideas what is going on with my trading post?

The reason you can't trade your items with the merchant is because they have a trade value of 0. Certain merchants won't accept certain kinds of good for trade, and so the value of those goods will be set to 0 when trading with them. An example of this is herbs, which a lot of merchants won't accept for trade, but food merchants will.

Merchants are traders and each merchant is looking for different goods. Having a wide variety of different goods on hand at the Trader Port is a must for productive (effective) trading. Items that are grayed out are simply not an item that particular merchant wants. The items that are not grayed out are what they are looking to trade for. Slowly build up inventory little by little as you have excess. Food is nice to have on hand for buying new seeds or trading for variety in diet if you don't have much variety. The higher end trade goods that bring in more are resources like stone, firewood, tools, clothing, ale. These are worth more in value. Food is worth 1 in value.