Just a small update...
I called the local ATF office in my state to clarify. He echoed the comments in this thread that the INTENT is the important part of the law. He gave an example of if you build one gun this year and sell it next year that's probably not built with the intent to sell. If you build three guns and sell them, you're probably manufacturing with the intent to sell. If you build a gun and sell it then build the exact same gun, that may be intent. Anyway, he said there's no number, its all arbitrary and their opinion. Basically its all common sense.
Now with regards to marking...
The agent I spoke with said he couldn't find any information where an individual who manufactures a firearm is REQUIRED to mark it. I asked him about the marking it "should it be sold or lawfully transferred in the future" part with regards to how an FFL would enter an unmarked weapon in their books. He said they would enter it similar to a gun made before 1968 when serial numbers weren't required and that my name would be listed as the manufacturer.
To sum up, and according to the agent, I can manufacture a gun for personal use. I can later at some point in the future decide I no longer want it and sell or trade it (so long as the intent when building it is NOT to sell it). I am not required to mark it in any way before sale (that part seems sketchy to me, but whatever).