The assault weapons ban is on the manufacture of assault weapons for civilian use. If you already have one, you're allowed to keep it. If you don't have one and want one, you'll be allowed to buy one on the market provided it was legal when manufactured (during a period with no ban and otherwise legal). The ban is only on the manufacture. At least, that is the ban that expired in 2004. I'm pretty sure Congress has the authority to regulate commerce.