Trading is very important in Patron. You're running a fresh new colony, ready to reap the rewards of your citizens' hard work. One way to earn your city a buck or two is taxes and that goes a long way, but trading with the mainland will get you farther. Trading is done through Docks and its upgraded version, the Harbor.
Naturally, you'll first have to build a Dock and assign a worker to it. A button will appear above the building. When you click on it, the Trade panel of this particular Dock appears. Notice the "particular" in that sentence. The more Docks you have, the bigger the trade volume you can shuffle.
Trading is done via ships, so it is important to notice that ships have a limited cargo space. They can't carry everything you have at a moments notice. Some planning is also involved here as well. The Docks start with smaller ships, only 200 cargo space limit but the they can be upgraded to accommodate ships with deeper draughts and carry more cargo. Though when you need the really big ones, the Harbor is your answer.
The correct way to look at the trading panel is like filling up a cargo manifest. You set any resource to a negative value, which means you'll be selling that amount and you can also set any resource to a positive value, which means you're ordering the resource from the mainland. The same ship will conduct the actual transport. Keep in mind that all Coin transfers are immediate, but you will be waiting for your ordered goods to arrive.
The Royal Fee
Another element to in the trading equation is the Royal Fee. The King takes a cut of any action, be it purchase or sale. And that same King can, and actually will, increase this fee over time. Each resources has an individual fee so it will take time for it all to go to any ludicrous heights. The Fee is always at your expense. When you're purchasing goods, you'll pay a higher price and when selling, the King will take a nice cut for the crown.
The Fee story doesn't end there, however. The King will notify you of his intent to increase the fee through his envoys. You'll have a chance to turn him down. The current fee, whatever it is, will remain but it will anger the King, each objection a little bit more, until the man-child completely loses it and announces a full Boycott of one of your goods. This means you won't be able to sell or buy the boycotted resource. Well, not on the mainland, that is. The Custom house upgrade of the Docks and the Harbor can circumvent boycotts and you'll be able to trade the resource via shady channels and at a significant price hit (lower price when selling, higher price when buying). Gladly accepting a boycott is not advised before the fee reaches at least 50%.