To trade you need to go to your harbourmaster's office and assign a ship to buy or sell goods of your choosing. To be able to trade, you need a pier, a trade storehouse built on the pier, and a trading ship (ship built with a merchant; if you're buying goods).
Trade only items
As you cannot produce them yourself, you have to trade for these goods. Use your trade menu, select the type of goods you want and then choose the tab “Trader”. There, you will find the offer from your friend Northburgh. But be careful, as even Northburgh does not produce these goods by himself and is dependent on his trading partners. This leads to fluctuating prices. If you want to get the best price, you have to check prices often.
Trade-only items include spices, indigo and silk.