Throughout the last chapters of the book, several clues led us to think that Mark was possibly doing something such as drugs. At first, Bryon notices that his brother doesn’t even spend a lot of time either with him or home. He even started bringing in some serious money, even though he didn’t have a job. Also, let’s not forget that he was the one who knew where M&M. This states that either he was hanging out with hippies or a more logic reason : he was selling drugs to them, since it is known that they consume them. He must’ve been doing it for a couple of weeks too, because as we can notice, he knew most of the people and the surroundings pretty well.