I work in the gun shows in the Washington, Maryland and Virginia areas. It is perfectly legal to purchase a firearm from an individual at a show if they are not a dealer. 20/20 implied that the person selling the Glock was a dealer by calling him a "seller". There was nothing illegal about the transaction.

The show did continue with their fictions when the person entering the classroom as the shooter, targeted the student that was holding the gun. With the camera on the shooter, you can see them go specifically to that person and ignore the rest of the class.