Every time the anti-gunners mount a new assault on our gun rights, people pile into the gun stores to get their hands on an(other) AR-15 while they still can. The problem is, a complete AR is expensive. The receiver is the part that is controlled though; everything else is just parts. So it stands to reason that if you might want more than one (now or in the future), it makes more sense to buy a few lower receivers and lower parts kits. The lower receiver itself is the only part you have to purchase through a licensed dealer. At the time of this writing they are well under $100 each (I've seen them for less than $50), and a lower parts kit consisting of all the small parts that go in the receiver is likewise well under $100.
This video is a complete tutorial of how to assemble those parts into a working AR lower. All you need then is a completed upper, which attaches to the lower in seconds to make a fully functional AR-15.