People who operate in the shadow market/economy

I have a family friend, he's a guy in his late twenties. Works as a personal trainer, and apparently films weddings. He did go to college to get an exercise science degree, just for reference. I always see him traveling, and doing stuff on social media.

What I'm really getting at, is, I basically don't understand how he affords his lifestyle. Doesn't come from a super wealthy family, he does live in his own condo, and I know bills add up. 

I know there are a lot of ways to make money, some more wholesome than others.

So I wanted to throw it out here, does anyone know someone who basically operates in a cash business, or basically something in a shadow economy, and survives off of it? Could be someone like successfully selling drugs, having a lawn care business thats all cash, or just cutting hair for cash.