Even if you by a stripped lower it gets registered. Only way around that is to by a lower from an individual. Also, if you buy a lower and an upper at the same time from a dealer they consider it a complete gun and there is about a 20% tax added. (maybe not all dealers, I'm not sure)
Not talking about a stripped lower like what you can get from spikes, noveske, rra, etc. When I say incomplete lower I mean these: