Actually, although you'll doubtless hear a lot of hysterical screaming about it, the actual verbiage stamped on the gun has no legal weight per se. Colt marks them thus to ensure that their distributors/dealers don't buy the specially priced LE/Military products (these lowers are marked "Colt Defense" rather than Colt's Firearm Mfg., etc.), then part them out or reconfigure and sell them as post ban guns. And as I understand it from looking at the relevant legislation, it is specifically illegal for a dealer to do that with a complete gun.
However, the lower itself is just another post-ban lower, since a lower by itself is not an Assault Weapon. Once a "LEO only" gun is purchased (by an agency or even another FFL) and leaves the distribution channel, it becomes just another gun, except that since it's configured pre-ban it can only be owned by a LE/Military agency or other entity. BUT, if that agency, individual or FFL then wants to sell the lower, it's perfectly legal for them to strip it down and do so. I have bought several stripped A2 and A3 LEO carbine lowers, had them transferred through an FFL quite legally, and stashed them away in my gun closet in anticipation of owning some neat collectibles after the AWB sunsets. And if it doesn't sunset, then I'll just build up a couple of post-ban guns and press on.
The guy selling the lower you mention (the LEO 9mm Carbine lowers on Gunbroker, I'm guessing) may have an ATF letter, but it's not necessary. It is in no way illegal for you to own this lower as long as you don't do anything with it to violate the AWB.
Again, a lot of people will tell you that you'll go straight to hell if you touch one (and it may raise a few eyebrows at the range so use judgement) but it's just not true that mere civilians can't own these.