Mr Moustache, you CAN get a Class 3 ATF license, but it's nearly impossible as an individual. Easiest way around it is to file a business license for a corporation, then have the corporation get the Class 3 license. Anyway...
As far as the 'lower reciever' questions, the majority of these sellers sell without, expecting you to go out, buy a standard semi-auto reciever, and put in a new sear pin to make it fully automatic. While remaining an illegal weapon, you are now the proud owner of a fully automatic assault rifle. Why you would want this is beyond me, seeing as how the majority of sear pin manufacturers do not adhere to original specs, and require a heavy duty firing pin and spring. In short, a 'standard' MI AR15 will put out around 400-600 rounds/min cyclic, and these 'homebrew' AR15's will put out around 700-1000 rounds/min cyclic. Why is less better? Imagine emptying a 20 round clip in 1.4 seconds with any accuracy. That's not to say the the original spec of 3.2 seconds/clip is much better, but al least you can hold it somewhat steady.
Another interesting side note, the M16A2's used by the army right now don't even have a full auto setting. only single shot and 3-round burst. Full autos are for jerkoff rednecks who shoot pop cans and street signs, and will probably die in a 'self-inflicted gun related injury' Ahem. At least, that's my opinion
