it is legal for a de resident to sell to another de resident without using a FFL transfer. HOWEVER, in my opinion, unless you know without a shadow of a doubt that someone is not a prohibited person, it's worth the time and trouble to make the buyer pay for an official transfer.
For me, when I've sold a firearm, it's only been to those folks I know very well, and I ALWAYS get a bill of sale written up with the S/N, make, model, and name & home address (as listed on their DL). I should say that the one time I did sell to someone I knew but felt like I didn't know that well, I made them pay for the FFL transfer. I also still completed the bill of sale.
Moosie