Most of my early money comes from selling prisoners to ransom brokers and the slaver in Tihr. To make it work you need a blunt weapon and/or a bunch of levelled up Manhunters. There is a maximum of 8 Ransom Brokers in the game. Talk to everyone you see, so later you can ask a Traveller where they are. Without you talking to them the Traveller does not know their locations.
Manhunters are weak, so they need help in levelling. I tend to hire Merc. Cav. or Horsemen for the job. Once they level up they do fine on their own. Sometimes I even fired the Merc. Cav. once they were not needed, so they didn't kill my bandits and didn't unnecessarily increase my party size. Small party is better for bandits.
Then I used to buy Enterprises in cities. Ask a Guild Master about them and go through the options. The second lowest is usually the best. It costs 10k and nets you 500-ish per week, and double that in Rivacheg. If you do that everywhere you will earn about 7k a week.