lower receiver assembly placeholder
This is the only AR-15 part that would legally require a background check in USA.
If someone has a 3D printer they could print one, though I don't know how well it would work:
http://www.thingiverse.com/thing:11770This^ .. there should be an open project to reproduce every part. Was thinking something like a glock would be easier to start with though.
I'm not aware of any 3d printing methods capable of producing a working lower. At least not working in the sense that you could use it more than once. I don't think that even laser sintering is up to it just yet. Maybe in a few years.
If you are going to do it yourself today, you want to go
here and read up while you wait for your Sherline to ship.