Sadly, a talented, honest young kid could make a killing running an underground garage to do typical servicing, because all the established shops/dealers are just so horrible. Of course he would be eventually found and and probably put in jail, judging from the attitudes in those areas.
I've actually thought about putting some posts up on craigslist at times when cash was thin to try and pull in some extra work... but better judgement kept me from doing it. It's not the fear of "getting caught", its more so the customers that you have to worry about.
Having been in the business, it's really not worth the hassle from people who will probably want you to install the cheapest parts from pep boys and then likely complain when there's an issue.
That's why i really only do work for friends/family (and sometimes local forum members that i've built up a history with). A quick example is my one buddy needed brakes done. I told him what to get and he came over. Well.... they didn't have the rotors i spec'd, they only had the cheap $20 front rotors and he also went with the cheapest semi-metallic pads they had. Before I even put them on i told him "they're going to make noise". Sure enough, a month later they're squealing like a pig
. For good measure (because he was concerned) i re-inspected everything and resurfaced his pads & rotors, and showed him that there was nothing "wrong"... so it's not a safety issue, just annoying. A month later the squeal came back, but he's living with it.
Difference between him and some random person; he knows im not screwing with him (especially since i did it for free). Some random person through craigslist would probably think it was my fault and would be demanding that i replace the pads/rotors for free for them. So the $50-$100 i might make just wouldn't be worth the trouble.
Another concern is when something completely unrelated goes wrong... like changing the fluid in the differentials and then the engine starts running rough. They have nothing in common but because of the 'timing' the customer thinks the engine running rough is your fault. Yeah... i've seen weird things like that.
And my personal favorite... someone who waits too long for a spark plug change and they're seized in the heads.
So yeah; it's really not worth the risk to open an underground shop, IMO.