There should never be a requirement on build-your-own guns.
A serial number stops no crime.
Criminals are already banned from having guns. Also, guns are illegal to use in illegal activities. A serial number does nothing, in these circumstances. If they are making them, the problem is that we have criminals making a gun they aren't legally allowed to have, with, or without, a serial number. If you can't trust a criminal to follow the gun law, punish and confine the criminal.
Our country requires a moral people to live in and run it. If we are not encouraging that, and letting in criminals, as every illegal actually is, then the problem is “us/US.”
The police need to do their job and the government should get rid of the criminals.
Banning kits or parts without a serial number you gave to government sources (they scoop them all up with visits to FFL and dealers), is unconstitutional.
WELL SAID & PLAYED!
TAXMAN BRAVO ZULU!