My 2% of a buck...
Back in the day...private transactions between friends...casual aquaintances...and total strangers was pretty commonplace. There were more guns sold in gun show parking lots than in the exhibit halls...and everybody knew it. My own father used to buy/sell/trade guns with the county sheriff. That's how loose things were...back in "the day" even after the 1968 GCA.
Nowadays, it's not quite as loose, and if there isn't an actual liability...it's assumed, and probably with good cause. I have a few close friends that I will buy/sell/trade with, but no more causual aquaintances, and no more strangers...period...without an FFL holder involved. No exceptions.
I know these people well enough to vouch for their character, their responsibility, and their sobriety. I have a few friends who drink to excess, and...although never charged with any assault type crimes, they're really not in full control of their sometimes volitile tempers when they're drinking...so I don't do any business with them, though they've asked me to "Be on the lookout for a good deal on a pistol for me."
Things have changed, and not for the better, I'm afraid.