From The Truth About Guns:

The ability to freely approach the gun counter to legally purchase a firearm is paramount to exercising the Second Amendment rights to keep and bear arms. There is no “keeping” of firearms if there is no legal right to lawfully acquire those same firearms. The ramifications of this flawed legal reasoning are self-evident. The government could simply ban the buying (and selling) of firearms and therefore eviscerate the Second Amendment all without infringing upon the right.