the merchants go the magical pixieland of pong where no man returns. i believe this has all been said before, so basically, summing it all up, the product must be in demand i.e. spices/slaves/silk for the country. it must be really far away from the capital, so a russian merchant would earn heaps more from chocolate in the new world than slaves in timbuktu. the merchant also needs skill, which doesnt come easily. i just pick out really weak merchants on the way to my destination and stay away from uber merchants. i also try to stop on products on the way, so the earn a bit. btw do merchants earn experience by reaching a certain level of gain, or by the time they sit on a product?