Dean, you could do that anywhere you want- however, you have a couple of hoops to jump thru.
First, assuming you are doing this to get paid for it, you have a business. You will need a business license, and be in a place where zoning permits you to run a business.
Second, you will need an 01 Federal firearms License (dealer/gunsmith) to do this IF the gun is left at your premises- just as a gun smith needs a license.
Part of that will be having secure storage for firearms.
I would also STRONGLY suggest you have insurance to cover your liability if it is alleged that you did something wrong that hurt a gun or hurt a person- and have insurance that will cover the "property of others in your care and custody". If I leave you a $2000 shotgun or rifle, I expect it back- and if someone breaks in and steal all the guns, I assure you, your homeowner's insurance is NOT going to cover that.
What we have heah is.... failure to communicate.