The more profit you make on a merchant the more experience you get for that merchant. First step is to click on your Trade Post and add items and/or gold. Items you can receive from the collection point and defeating various world mobs and lairs. After you have added items and/or gold you should click the "Leave" button.
Once you leave you need to get to a Trade Center to be able to sell. You can buy at villages but not sell.
Some people say that shorter distances work better because it is faster while others say that it's more profit longer distances. A long route that I see most people using is Rome where you buy Western Swords then head to Babylon (Persia) and buy Water Buffalo followed with the last destination as Changan (China) to buy Silk and return to Rome. You can look for the route that suits you the best.