Look at the lower carefully also. I doubt they would sell M-16 lowers (check out pics to see the difference) but you can never tell. Look carefully at the parts in the fire control group. Again, I doubt they're selling real M-16 parts in the lower.
If you like the looks of the early Vietnam era M-16, but of course just want a legal semi auto version, CDNN has some brand new Colt SP-1 AR-15 uppers at a pretty good price ($369). You'll need a bolt carrier assembly and charging handle (which I wouldn't buy from CDNN) and a lower. They claim they're brand new.
Normally I don't buy anything serious from CDNN, but they are probably the best scroungers around and where they found a warehouse full of Colt SP-1 uppers is anybody's guess.
Be very careful of M-16 parts though. They could land you in the fryin' pan with the friendly folks at BATFE.