Which particular clause of Article 1, Section 8 is the Montana gun-seller concerned with?

Clause 3 called The Commerce Clause.

Explanation:

Article 1, Section 8, Clause 3 of the Constitution of the United States notes that the U. S. Congress has the power to control trade with States, foreign nations, and the Indian Tribes. Montana gun law places the state at the forefront of a nationwide effort to reclaim the rights of States by targeting the influence of federal government up to a century of federal court rulings. Montana is the first state to pass the Firearms Freedom Act, which specifies that firearms produced and sold in the State are excluded from federal gun restrictions to buyers who intend to keep the weapons in the State.