I have seen it on various Internet forums. I too would question the authenticy of forums but it seems to be pretty wide spread.
Here is one i found by googling:
http://www.firearmstalk.com/forums/f...legality-2894/
There is a letter from the BATFE in there in which it states.
"Also, for your information, a nonlicensee may manufacture a semiautomatic rifle for his or her own personal use. As long as the firearm remains in the custody of the person who manufactured it, the firearm need not be marked with a serial number or name and location of the manufacturer. However, if the firearm is transferred to another party at some point in the future, the firearm must be marked in accordance with the provisions set forth in 27 CFR § 478.92 (formerly 178.92)."
So if you intend to sell the weapon you must mark it like any other weapon bought from a major manufacturer with serial number and everything.
Here is something a poster added to that:
"You may (will) run into problems if you were to build an unreasonable number of such guns in any given year and subsequently sell them : a hobbyist cannot make "a full-time income" from selling guns be they home made or commercial. Even if (as a hobbyist / gun enthusiast) you were to "buy / trade and sell" commercially made guns and then sell a few of your items at gun shows you must not make sufficient income for it to be classified as being a full-time income: this is the province of the FFL who is expected to make such an income. The ATF will ask to see records of ALL transactions you have made to establish what income you have earned."
So it is legal to sell the weapon just not in quantities large enough to be seen as manufacturing weapons for sale.