Just my $.02, but it also matters what you mean by "refurbish." If you mean just cleaning off the cosmoline and dirt or maybe replacing a part, I doubt you need a license. But if you alter the rifles, or "sporterize" them, then Hkmp5sd is correct, you will need a manufacturers license, since you are making "new" guns.
Of course there is also the question of where you get the rifles and how you sell them. Buying and selling guns, even for minimal profit, requires a dealer's license; doing it without the license or doing on a C&R license would be illegal.
Jim