I have experience in monetizing and selling companies both based on paid memberships to a forum and online courses. In both cases the value derives from you or your brand driving the purchase. So, you can't just create a course or private message board about, say, how to fix BMWs. There has to be a reason why they want to get the information from/through you. That's what will really drive the purpose. So, the passive income won't start out as passive, but can be once you have 1000 true fans (so to speak).