Are there any stipulations that I should be weary of due to being a Business Owner while in the National Guard?

Being a Sole Proprietor or Owed Operator of a Business is hard on Guardsmen. All the legal protections are gone, since you are the Company. I would be Leary of it, as you are it. Same as farmers that are in the Guard.

As far as stipulations, youll need the FFL and all the appropriate licensure. Your record keeping as a gun dealer will be very stringent. If you screw up and the government prosecutes, it will impact your Guard contract. You could be separated and or lose your security clearance. Also indebtedness can impact security clearances and personal reliability programs if so enrolled. Is it possible? Sure. Don't screw up. Good luck in your business.

Talk to your chain of command, but I can't think of a reason why you can't. If the Ft Bragg IG says a joe can swing on a brass pole for moonlighting , I don't see why you can't do this.

You need to be cognizant of what happens to your business if you are called up.

Is this small business your sole source of income and do you have a plan for this business survive you being gone for a year for deployment?
That's the first question you need to answer for yourself.