There is no federal law nor ATF regulation requiring a serial number on a firearm you manufactured for personal use. There is also no federal law nor ATF regulation requiring a serial number on said firearm if you later decide to sell it. It is illegal by federal law to be in the business of manufacturing and selling firearms without a license. I have a post from another thread with links to pertinent pages on the ATF website, here. It did take a bit of querying to find the appropriate information to post that the first time. The amount of false claims about personally built firearms and serial numbers and selling them made me do the research. I personally do not care whether or not someone builds a firearm and what they do with it afterwards, and neither should our government.
As to the discussion on SBRs and bump stocks, as several pointed out, that, too, is a non sequitur, as it really does not matter what type of grip, stock, barrel length a firearm has as to it being used in a crime. As addressed, murder, robbery, etc., are the crimes, the firearm is not the crime. Also, the ATF definition of what a pistol is, is interesting, here, as it does not mention length of barrel, nor anything else, other than grip must be below the bore, and the firearm is designed to be held in one hand. So that makes the whole argument about barrels, etc., moot.
One other interesting point on the silliness of ATF regulations pertains to cannons. I have a link to the pertinent ATF page for that, too, in the above link. A muzzle-loading cannon, built before 1898, or replicas, that I would believe no one would claim to not be capable of horrific destruction, is not classified by the ATF as a destructive device, but a breach-loading cannon, regardless of size (apparently), is a destructive device. One has to love government logic.