I was under the impression that all the parts of a full auto gun except the reciever were legal to own, import, and sell (as long as you don't also own a receiver that they could be potentially used in). They are advertised widely on the internet and in various gun publications.
I remember a few years back when a bunch of legaly imported AK47's were found to be full auto recievers that had just had the semi auto parts installed (some extra holes drilled in the receiver or something). The ATF recalled the guns and tracked down the purchasers of all the ones that had been sold. Wonder if this could be something similar?
Be careful -- National Firearms Act, 26 USC section 5845, DEFINITIONS-- defines "machinegun" to include "...any part designed and intended solely and exclusively, or combination of parts designed and intended, for use in converting a weapon into a machinegun,...."(bold added)
Source -- Gun Laws of America, by Alan Korwin.