I'm curious to know what the official answer is on this. I don't know for sure, but I don't think there's any paperwork for the seller to fill out in a private sale. I don't think you need to do anything except take the firearm to an FFL and fill out the buyer's form. I've done that before, but maybe the rules have changed. I suppose it can make a difference who the relative is. Maybe it's different for father to son or brother to brother than it is for uncle to nephew or unrelated people.
"When you put a buck on your wall, it links you to those guys who painted the walls of caves." --Me.