Geez. I recall it is already illegal to sell a gun w/o serial number to anyone else.
Arm brace pistols have been used in how many deaths?
'Ghost guns' have been used in how many deaths? (no serial number on lower receiver as home made)
Printed guns have been used in how many deaths?
My guess is zero.
Last I heard, printed guns were huge, awkward, and likely to kill the shooter. There was a fun episode of the good wife involving a printed gun. From memory, some guy down loaded a file and printed a hand gun. He takes it to the range, fires it, and it blows the face off the guy near him. Who to sue? Everyone. The file uploader, the downloader, the printer mfg, and the polymer mfg.
Making your own lower receiver does not require a serial number right now. How many people make their own lower receivers? If you make one it is illegal to sell without a serial number.