I'm confused... What exactly is wrong with the guy's auction, other than he has to come up with a clever way to word the auction such as not to violate ebay's policies? Ebay doesn't allow short barrel kits/parts, so he attempts to throw them through a loop by mentioning a 16.5" length from f/s to charging handle - likely their CS has been informed that 16.5" is the shortest barrel allowed, and all they look for is "16.5"".
I think its pretty obvious to any gun owner what is being offered, just like all the "10 round" magazines being sold. The upper is completely legal to own by itself - as long as you don't own an AR lower, or provided you have a registered NFA lower. Hell, it would even be OK if you had a preban AR pistol. I don't think it is any sort of "sting" as people have suggested, but rather some guy trying to get around EBay's silly policies on otherwise legal items.
Rocko