When someone presents a transferable as a NIB gun, I am very skeptical. I would never pay extra for something is NIB.
It it is way too easy to fake something NIB.
I remember for the longest time, a well known dealer was trying to sell a pair of NIB M16A2 for $39,000 each. I asked him how he knew it was NIB, and his reply was that they were still sealed in the bag. I called around to some guys who were armorers. No one remembers new M16s in the military arriving in bags. It's my suspicion that the bags were there to make it hard for the buyer to inspect the guns, and since the buyer would think the guns were NIB, they then would never open the bags to inspect the guns.
Its very easy to fake something NIB. Especially a modular gun like the M60. All they do is strip the gun to its registered part, and then put NOS on the gun, apply some oil, let it sit outside for a bit to obtain some old wear and patina and to even out the aged appearance, and voila, a NIB never fired gun. Or even easier is to send the entire assembled gun to US Annodizing, then no one will be able to tell. Heck they can even make it look like NOS, but in immaculate condition.
Same thing with an M16 or really any MG.
When MGs like this come up for sale, it hurts the credibility of the entire market because it just looks shady.