There is no "build law", only a lack of laws prohibiting it. ATF changed over to the Justice dept, are you looking on the old Treasury website?
In a nutshell:
You can build a normal firearm for your own use. Normal means no NFA stuff like short bbl or full auto, if you can legally buy it from your local gun shop with just a 4473 fillout you can build it.
You are not required to mark it in any way, although ATF "strongly recommends" you put at least a serial # on it for tracing purposes should it ever be stolen. You are not required to file paperwork or report it in any way to anybody. If you start out with a commercial receiver, it is already legally marked. There is no excise tax due as there is an exemption for personal use firearms.
Firearms that are banned from import are not allowed to be built with more than 10 foreign parts from a list, commonly known as "922(r) compliance". If you wanna build an AK, gotta put some US parts in in to be legal.
That's pretty much it, that only addresses Fed law though. State and local laws vary.