To add context, you can't just go to a gun store and buy a gun at the age of 13. Federally licensed firearms dealers are required to perform a background check prior to selling a gun to anyone. Private sales from a non-licensed individual however allow the sale of a long rifle or shotgun to a person of any age. Pistols are still restricted by age in either case. An individual must become a licensed firearms dealer to sell firearms for a profit.

What you're seeing in this video is what's commonly referred to as the "gun show loophole" where private sellers go to a gun show (due to the number of potential buyers) and sell their weapons. If the individual that sold that rifle to the kid made a profit he technically did break the law, otherwise this is all okay by the books currently.

An interesting topic for discussion but I believe the context above is necessary.