All the NFA guns I've bought were from a reputable Class dealer and or a reputable seller that I knew.
I've also used contracts and notaries on the high dollar purchases as well.
Buying title II / FA firearms is truly a leap of faith. The Martinsburg branch appears to be turning around
Form 4's quicker these days.
I would suggest that anyone looking to take the leap, to do lots of research and join a club that specializes
in the firearm you're looking to buy, as in the Thompson Collector's Association for instance.
You'll be glad you did.