Gun Sales Within a State

What gives Congress authority to regulate the sale of guns when both buyer and seller are within the same State?

When the Supremes handed down a decision in The United States versus Miller in 1939, upholding the National Firearms Act of 1934, they concluded that it (the NFA) was not an unconstitutional invasion of the reserved powers of the states. They cited Sonzinsky v. United States, 300 U. S. 506, and Narcotic Act cases. P. 307 U. S. 177.

Inferentially, they referred to Article One, Section Eight, 16th paragraph, which reads, in its entirety:

"[Congress shall have the power] To provide for organizing, arming, and disciplining, the Militia, and for governing such Part of them as may be employed in the Service of the United States, reserving to the States respectively, the Appointment of the Officers, and the Authority of training the Militia according to the discipline prescribed by Congress;"

The referential portion of the decision reads:

"In the absence of any evidence tending to show that possession or use of a 'shotgun having a barrel of less than eighteen inches in length' at this time has some reasonable relationship to the preservation or efficiency of a well regulated militia, we cannot say that the Second Amendment guarantees the right to keep and bear such an instrument. Certainly it is not within judicial notice that this weapon is any part of the ordinary military equipment, or that its use could contribute to the common defense."

Democrats like to ignore the Constitution.

What gives Congress authority to regulate the sale of guns when both buyer and seller are within the same State?

Federal law trumps state law. It's in the freaking US Constitution.

Nah. We like the odd numbered amendments. You like the even ones.

I prefer the Fifth.

The Second Amendment, which is often cited by people who don't want gun control, references the militia.
Article One, Section Eight, 16th paragraph also references the militia but the same people don't cite it.

Your original question was related to jurisdiction. The Second Amendment is a fundamental human right.

...a fundamental AMERICAN right.

Our "god-given-rights as god-damn-Americans" doesn't apply outside our border