KAC BUIS - real or not?

[omega21]

So I was planning on buying a rear BUIS from a member here, and I was just checking prices on line. Most everyone sells them for well above $100. When I was looking at eBay I see several that are marked (on the sight) "USMC" and seem to be selling in the $50-80 price range. Is this some kind of knock-off? Kind of like the MAGPUL sights made for airsoft (I know, those were made by Magpul, not knockoffs).
Also, is there any easy way to distinguish between a KAC real vs knockoff iron sight? (via pics, I'd rather not have the fake in my hands to make that discovery). The people selling on eBay are US based and have 100% good feedback, so I'm not talking about any "Hong Kong Specials".

[JBecker 72]

The USMC marked sights are stolen gov property (plenty of info here and on TOS about them). If you are concerned about buying a fake KAC product, don't buy it on ebay, buy it from a reputable source like www.blotcarrier.com or www.operationparts.com or Knights Armament directly.

[scoutfsu99]

They've been mentioned several times here. They're real but misappropriated from the USMC.