I had two Bushmaster rifles that I purchased used from the consignment rack at the famous B&B in North Hollywod. Out of curiosity, in the mid-90's I wrote a letter to Bushmaster and requested any information about the date of manufacture. Not only did they provide the date (Both were made in 1994, both before the Federal ban), but their records show one left the factory as a complete rifle ("V-Match 20" with bayonet lug and muzzle threads with flash hider) and the other left as just an assembled lower with fixed stock.
As long as the end user built it into a preban configuration before the ban took effect, you were (Federally) legal. Obviously, manufacturer's records that is was built as a complete weapon with the certain features would be more than enough proof. For the lower receiver sales, I can only guess how you would prove it (Dealer statements or receipts that indicate the configuration is was built and sold as; Witness statements, date-stamped photos... )
If you are a Team Member here, I remember there was lots of discussions on the subject if you want to try to search the archives.