It's no different than owning a short barrel for a rifle but not owning the rifle. It's not illegal until it's with it's parent firearm.
Notice their warning about "Constructive Possession". If you own that sear kit AND a Glock handgun, you'd be in "constructive possesion" of an illegal full-auto gun.
Own the kit without the gun, no problem.
So you could, but why would a non-LEO even want one? What's the point?
"Courage is being scared to death and saddling up anyway." - John Wayne
.44 Special: For those who get it, no explanation is necessary. For those who don't, no explanation is possible.