The Selfish interest skill can be easy or difficult to master depending on the personality of the hero.
While evil heroes find that this skill is the easiest to be mastered, good heros find themselves struggling with their sense of morality while attempting to exercise this. Mastery of the skill allows the hero to drop all concern for the needs of the shopkeeper at will, allowing him to plunder goods without concern. Additionally, stronger heroes find that the trader becomes more compliant with their demands while using this skill.
At this point, the hero will occasionally give a few fake coins to the trader, allowing him to keep real ones. The merchant will be unable to see that such forgeries exist.
The hero will frequently argue with the merchant about the price of items, ensuring that he gets the best possible deal. Much of the money the hero gives will be worthless mud instead of gold, although the trader, wishing to avoid another argument, will pretend that the mud coins are actual gold. He may also steal small amounts of money from heroes or other merchants.
The hero will be willing to use more extreme mechanisms to get whatever items he wants, such as threatening to beat the merchant unconscious if he doesn't comply with taking several thousand coins off the price tag. Telling the hero about needing to feed a starving baby will be ineffectual, and only feed his already strong skill.
The hero loses all sense of morality and will steal all goods for free without even talking to the merchant. At lower levels of this, the hero will throw the merchant out of the shop in order to spare him the sight of losing all his goods, but this behaviour soon dissapates and the merchant is forced to watch in horror.