Wickard v. Filburn
"Whether the subject of the regulation in question was 'production', 'consumption', or 'marketing' is, therefore, not material for purposes of deciding the question of federal power before us.... But even if appellee's activity be local and though it may not be regarded as commerce, it may still, whatever its nature, be reached by Congress if it exerts a substantial economic effect on interstate commerce and this irrespective of whether such effect is what might at some earlier time have been defined as 'direct' or 'indirect.'"
I doubt if private sales of firearms will "exert a substantial economic effect on interstate commerce".