Stealing is by far the most lucrative method to earn money quickly in the game. Before trying to steal, make sure you have unlocked the ability to sell stolen goods. It is also recommended to get a horse to speed up travel times. The quieter towns with guards are a good place to start. Wait until dark and approach a guard from behind. Knock him out, loot all his gear, and sell it to a miller. Bailiff properties and noble residents usually contain very expensive items, such as goblets and plates -- all of which sell for a good amount of money. This works best when combined with haggling. Other good targets include armories, barracks, and stores. Scattered throughout the game are various contests, such as the archery competition in Rattay. While you do not get a ton of money from it, the higher ranked tournaments do pay really well. It is also legal, so you do not have any of the risks or negative side effects of the thief lifestyle.