What does the serial number accomplish for you? (I mean, yes, when you get a gun in your possession at a crime scene, you can backtrace it through to the original point-of-sale, but what does that accomplish exactly? How many Bad Guys have they caught from having a gun in-hand and tracing it? )
A letter I have here from the BATF clarifying whether it was necessary to put a serial number or marking on my projects says:
"Additionally, although markings are not required on firearms manufactured for personal use (excluding NFA firearms), owners are recommended to conspicuously place or engrave a serial number and/ or other marks of identification to aid in investigation or recovery by State or local law enforcement officials in the event of a theft or loss of the privately owned firearm."
Firearms manufactured for sale are regulated a bit differently than ones made for personal use.