If you work on guns or their components for a profit you are engaged in the business of gunsmithing, whether or not the transferable item is in your possesion or not. Technically you could not even finish a stock for profit unless you have an FFL. Most of the time these little infractions go unnoticed, but I would not make it a point to publicly advertise offering those type of services without an FFL.
Good luck explaining yourself to an ATFE agent.