Might be true in California, but this is almost entirely false at a federal level.

You can't make it for the purposes of sale, but you can sell or loan it as part of trading your personal collection. I've heard the myth about not being able to sell over and over but no one has ever been able to point out a federal law against selling a privately manufactured firearm incidentally later as part of trade in their collection, with or without a serial number. All successful prosecutions I've read involved people making them for the purpose of sale or transfer and then getting caught doing that -- for that you need an FFL.

You do not have to be a FFL to transfer a nonserialized firearm. In fact tons of guns made before the GCA had no serial number, as there was no blanket requirement before 1968, they are legally sold privately all the time (as are PMF / "ghost guns" that people no longer want).

>[addndm] "Requirements for Individuals

Yeah that's an uncited bit of misinformed nonsense, it's totally false. If there is a law or ruling they surely could have cited it, in fact what they did was apparently trawl forums or something repeating that myth and just regurgitated it out. Here is the actual rule they claim they are referring to[] I challenge anyone to find that nonsense in there.

In fact, it says the exact opposite, as I will cite the actual rule publication that those morons are pretending to refer to but yet won't cite themselves:

  At the same time, neither the GCA nor the proposed or final rule prohibits unlicensed individuals from marking (non-NFA) firearms they make for their personal use, or when they occasionally acquire them for a personal collection, or sell or transfer them from a personal collection to unlicensed in-State residents consistent with Federal, State, and local law. There are also no recordkeeping requirements imposed by the GCA or the proposed or final rule upon unlicensed persons who make their own firearms, but only upon licensees who choose to take PMFs into inventory. In sum, this rule does not impose any new requirements on law-abiding gun owners.