Well I'll just throw this in for whatever it's worth.
IF the upper is off of some units weapon NOBODY has the authority to sell it. When the unit the upper was STOLEN from DEMOB's its weapons will be processed through DOL (Directorate Of Logistics) weapons section and if that weapon has anything other than a military issue upper it WILL be noticed. They're going to do a 3-shop/depot level inspection on every serial number on the Sensetive Items List and if one (Original upper) comes up missing there's gonna be ALL sorts of "Fun" for the people involved (LEAST of all the poor SOB who's signed for the weapon and his unit armorer).
In situations like that people start back-tracking and trying to figure out where the THEFT occurred. Your buddy's buddy might find himself having to talk VERY fast (Naming names) to avoid having to spend some "Quality time" in Kansas OR (If he's covered his tracks well enough and Buddy-Fucked someone else) he'll leave some other poor kid (Who, BTW, just spent a tour in IRAQ serving his country) holding the bag.
Lastly, I don't know about civvie KAS rails but those in the military ARE seriel numbered and carried on the Sensetive Items Inventory (Meaning, they're traceable unless you remove them).
I just went through a De-Mobilization 3 1/2 months ago and was the NCO who walked our unit armorer through the DOL inspection.
Do what you think is right dude but if you even SUSPECT that the upper is STOLEN and STILL go for it ?