Ok, here is the source of confusion. From Articles/Legal/Drop In Auto Sear on this site, I quote:
"Possessing an unregistered post-November 1981 autosear is possession of an unregistered machinegun. Possession of a pre-November 1981 autosear is simply possession of a 15 piece of steel unless you also possess an AR-15 with M16 parts. Then you are also in possession of an unregistered machinegun."
From the way I read it, a post 1981 sear itself is considered a "machine gun" and must be registered with the BATF, with or without an AR 15 nearby.
As I mentioned, there was no mention of legalities on the order form. Just list the parts you wanted and your credit card.
BTW, thanks for the responses to this new member. As usual, most gun owners prove themselves to be friendly folk...