Scamming people, running a pyramid or ponzi, selling shovels.

edit: I'm only semi-joking, the official Bitcoin forums quite literally has pyramid and ponzi and straight up fraud schemes that people happily sign up to and refer each other to. There is no attempt to hide them or look legitimate. You'll find ridiculous-to-outsiders threads asking "hey anyone know a good ponzi i can sign up for will use your referral"

The marketplace subforum is nearly totally populated with stolen/hacked accounts, subscriptions using fraudulent/stolen credit cards, it's pretty much a fraud forum.