Just to ad fuel to the fire - The ATF estimates there are an average of 266,000 firearms stolen in the US each year.
I cant give an actual, verified number, but my experience in LE has been that a fairly significant percentage of those end up in crimes, even if the perpetrator was not the person who originally stole it. And there are quite a few folks, who for whatever reason, do not report firearms theft to the police.
R/E the original topic, as a non-licensed individual, you are still allowed to manufacture a firearm from scratch for personal use (non-resale), and I don't believe a serial number is required.
Larry