Trading is a reliable source of income, especially in the early game. The basic principle is to buy goods cheap in one town and sell them for a higher price in another. Pay attention to the prices of goods in different regions; for example, iron is often cheap in Nordic territories and expensive in Swadian lands. Tools, salt, and raw materials are generally good trade items.
Once you have accumulated some capital, you can invest in Enterprises. These are businesses you can establish in towns (e.g., a Dye Works, Ironworks, or Brewery) by speaking to the Guild Master. Enterprises provide a weekly passive income, making them an excellent long-term investment. The profitability of an enterprise depends on the town's prosperity and the availability of raw materials. Aim to establish enterprises in prosperous towns with good access to the necessary resources.