"A 2011 directive from the Bureau of Alcohol, Tobacco, Firearms, and Explosives specifically prohibits federal firearms dealers from selling to marijuana users, both medical and recreational. Private gun sales between individuals, which are not federally controlled, do not have this restriction."

This is false. All restrictions on firearms acquisition and possession by Prohibited Persons apply to private sales just as they do to FFL sales. That people can break the law and -ignore- the rules in either kind of transaction doesn't mean the regulations do not exist. 

It is a peculiar weakness of the authoritarian and submissive mindsets when it comes to guns that they think that if an action can occur despite the existence law that action is somehow "legal."