Biggest issue I see with side work for pay is liability - real, legal liability. If you fix an engine (regardless of what you repair) and it stalls out for whatever reason, causes an accident of any kind who are the attorneys coming after? Or you replace a shock, ball joint, whatever, and the vehicle is in a wreck? Attorneys are looking at who worked on the vehicle and when.
Doesn't matter what you did, how well you did it, and whether or not it contributed to the accident/wreck - you most likely will be served and will need to defend yourself in court. Forget about any non-business insurance you have - it won't cover for this if you accepted payment.
Why do companies charge so much for work? Yes, of course they need to make a profit to stay in business. But also to pay for insurance, business taxes, income taxes, property taxes, health insurance, employee pay, etc.
With my company, our three biggest expenses in order: Employee pay, taxes (Income, FICA, property, etc), insurance (liability, health, etc). Depending on margins within the specific industry this doesn't leave a lot on the table for the business itself. Just look at the prospectus for any specific company stock and they spell it out right there. Easy to see why things cost what they do, and what potential liability costs all of us.
On several welding forums I read, smaller guys form LLCs, work at "their" company with no liability insurance to cover mistakes or errors (errors/omissions insurance), and when sued declare bankruptcy and move on. I have some acquaintances who paint, and they do the same thing. Close the old business and form a new business when the old one gets sued. Attorneys have no one to pursue. LLC = limited liability company
Anyway, it you want to work on the side, have at it. Just cover your bases or you might be sued for all you have if something goes wrong - even if it's not your fault.