It is not illegal to make a Title I firearm for your own use. You can even make an NFA (Title II) firearm (other than a machinegun) for your own use, if you pay the tax and get approval.
It is illegal to make a firearm for sale of distribution unless you have a manufacturer's license. But it is NOT illegal to sell a firearm you made for your own use as long as you didn't make it with the intent of selling it.
How would BATFE show your intent? I am not going to get into all the "scenarios" but it is usually pretty easy.