Come off it, Revitup! You asked how you could do something legally that you knew darned well you couldn't do legally, then get all upset and sarcastic when people tell you that.
If you want an untraceable gun, probably stolen, with the numbers ground off, I can assure you there are plenty within a few miles of where you live. But they aren't legal. So make up your mind if you want an untraceable gun or a legal gun. You may be betting your future freedom on the answer.
As for "Federal Registration" there is none. The NICS check does NOT involve the make, model or serial number of the gun, only whether it is a long gun or a handgun. Yes, guns can be traced through dealer records, but if you think that buying from an individual will mean an untraceable gun, you are in neverland.
Besides, if the day ever does come when the state or feds decide to use the Army or national guard to seize guns, it won't matter whether one gun is traceable or not - they will just take them all.
Jim