“For the power given to Congress by the Constitution does not extend to the internal regulation of the commerce of a State, (that is to say of the commerce between citizen and citizen,) which remain exclusively (emphases added) with its own legislature; but to its external commerce only, that is to say, its commerce with another State, or with foreign nations, or with the Indian tribes.” –Thomas Jefferson, Jefferson’s Opinion on the Constitutionality of a National Bank : 1791."State inspection laws, health laws, and laws for regulating the internal commerce of a State, and those which respect turnpike roads, ferries, &c. are not within the power granted to Congress (emphasis added)." --Gibbons v. Ogden, 1824.
As a side note concerning ATF, please consider the following. Regarding agricultural products like alcohol and tobacco, before Constitution-ignoring socialist FDR nuked the Supreme Court with activist justices, Supreme Court had clarified, in terms of the 10th Amendment nonetheless, that Congress has no constitutional authority to regulate intrastate agriculture.
"From the accepted doctrine that the United States is a government of delegated powers, it follows that those not expressly granted, or reasonably to be implied from such as are conferred, are reserved to the states, or to the people. To forestall any suggestion to the contrary, the Tenth Amendment was adopted. The same proposition, otherwise stated, is that powers not granted are prohibited. None to regulate agricultural production is given, and therefore legislation by Congress for that purpose is forbidden (emphasis added)." --United States v. Butler, 1936.
Sadly, the reason that the gun shop owner is letting the ATF walk all over him is likely the following. Like most other citizens, the proprietor's parents/guardians probably did not make sure that their son was taught the Constitution and its history in school, particularly Congress's Article I, Section 8-limited powers.
Even more sadly, it wouldn't matter either way, because his judge, jury, and prosecutor weren't, and they couldn't care less, since it would get in the way of almighty legal precedent.