"Yeah, I called NICS" doesn't work for me. I'd be fine with making *all* transfers go through an FFL. With the sole exception of my best friend whom I've personally known for over 25 years, every gun I've ever sold has gone through an FFL so I don't have to answer any questions if a weapon I once owned turns up at a crime scene.
Hell, create a Type 4 license solely for mediating transfers - not allowed to maintain any physical inventory or perform direct sales, just call in the check and keep a bound book. It creates a full chain of custody record and would make a perfect cottage industry or bit of extra income for retirees and such.