You can legally buy and own that sear, you just can't own an AR15 if you do.
No, you can not. You would be in possession of an illegal machinegun. The same thing applies to H&K trigger packs that are select fire as was covered in the legal section of Small Arms Review a few months ago.
From the ATF web page:
http://www.atf.treas.gov/firearms/faq/faq2.htm#m1
(M1) The types of firearms that must be registered in [Back]
Some examples of the types of firearms that must be registered are:
Machine guns;
The frames or receivers of machine guns;
Any combination of parts designed and intended for use in converting weapons into machine guns;
Any part designed and intended solely and exclusively for converting a weapon into a machine gun;
See this part here? This means an auto sear and the mentioned trigger packs. They have already successfully prosecuted people for owning just these parts. The auto sear is designed soley for the purpose of converting the weapon to full auto.
Any combination of parts from which a machine gun can be assembled if the parts are in the possession or under the control of a person;
Silencers and any part designed and intended for fabricating a silencer;
Short-barreled rifles;
Short-barreled shotguns;
Destructive devices; and,
"Any other weapon."
A few examples of destructive devices are:
Molotov cocktails;
Anti-tank guns (over caliber .50);
Bazookas; and,
Mortars.
A few examples of "any other weapon" are:
H&R Handyguns;
Ithaca Auto-Burglar guns;
Cane guns; and,
Gadget-type firearms and "pen" guns which fire a projectile by the action of an explosive.
[26 U.S.C. 5845]
Then there is this:
(M17) Are parts which would convert a firearm into an NFA firearm subject to registration? [Back]
Yes. Examples:
An M-2 conversion kit (See QuestionM29);
Any part designed and intended solely and exclusively to convert a weapon into a machine gun. (See Question M1.)
This means that the auto sears need to be registered. Period. If they are not, they are controban.
And there is this:
(M29) What is a "conversion kit?" [Back]
A conversion kit is any part or combination of parts designed and intended for use in converting a weapon into a machine gun. A conversion kit is a machine gun. for purposes of the NFA. (See Question M17.)
[26 U.S.C. 5845, 27 CFR 479.11]
So someone please explain to me, and show evidence that these can be lawfully purchased and transfered to an individual without paying any taxes or being subject to the 86 ban on the manufacture and civilian possession of post 86 production machineguns and the parts to convert semi auto weapons to machineguns.