Unfortunately, the days of doing small money making gunsmithing jobs are over.
Today, you'll almost certainly have to have a Federal Firearms License as a gunsmith. (I know, you aren't really gunsmithing, but the BATF says you ARE, and you have to have a FFL license). This means that if you intend to receive a gun from out of state OR if you keep the gun overnight, you are required to have an FFL and keep an FFL record book to show the BATF when they inspect you.
If you have a FFL you also have to have a state tax number, and any state or local licenses or permits.
You also have to have your "place of business or shop" in an area that zoned for a business.
Locally, there was a man recently who wanted to do stock refinishing to earn some money on the weekends.
He was turned down because the neighbors claimed his house wasn't zoned for business use.
Next, you BETTER have insurance.
Do nothing more than a polish job on someone's gun and if he has any kind of problem at all, even if he shoots himself in the foot, guess who gets sued?
You.
Not to rain on your parade, but things have radically changed over the years and today, you have to meet all the requirements, or you're committing a Federal felony crime.
The days when you could do stock work, re-bluing, or gun polishing without literally making it a Federal case are over.