You will have to have an FFL, and if doing business in IN will have to have a Tax ID.
From the ATFE website
Q: Is a license needed to engage in the business of engraving, customizing, refinishing or repairing firearms? Yes. A person conducting such activities as a business is considered to be a gunsmith within the definition of a dealer.
[27 CFR 478.11]
Q: Does a gunsmith need to enter in a permanent “bound book” record every firearm received for adjustment or repair? If a firearm is brought in for repairs and the owner waits while it is being repaired or if the gunsmith is able to return the firearm to the owner during the same business day, it is not necessary to list the firearm in the “bound book” as an “acquisition.” If the gunsmith has possession of the firearm from one business day to another or longer, the firearm must be recorded as an “acquisition” and a “disposition” in the permanent "bound book" record.
Q: Is an ATF Form 4473 required when a gunsmith returns a repaired firearm? No, provided the firearm is returned to the person from whom it was received.
[27 CFR 478.124(a)]