Thanks, Colt808. I try not to make too many assumptions when reading the questions, but apparently not everyone is like that. If you get into the "if you're in Hawaii, or if you're in California, or if you're in Oklahoma..." the answer becomes too complicated, and usually unnecessarily. I boil it down to the federal requirements, then the state laws depend on where you live.
The overall answer I was trying to get to was this: KNOW THE FEDERAL ATF LAWS AND THE LAWS OF THE STATE WHERE YOU LIVE. If in Hawaii, the handgun seller must sign the Permit to Acquire. I, however, would still ask for the guy's ID, just to verify for myself the permit holder is the same guy in his driver license picture. You never know if the purchaser can't or won't meet in person, so he sends his buddy to do the deal. Some things are simply due diligence when meeting someone for the first time. Ask for ID and take a pic with your handy cell phone. There's no excuse for being too lazy or shy to ask for ID. If I've met the person before, then I'm not worried about identity.
There is no law that requires us to use common sense. I prefer to take the more prudent stance as a responsible gun owner (and to cover my own butt) versus doing only the minimum the law specifically states.
As we've discussed in these forums many times, it's up to the buyer and seller to agree on the details of a sale, including form of payment, meeting places, and what paperwork and permits must be created/presented or signed (bill of sale, permit, copy of ID, etc). As long as the legal minimum requirements are covered, the rest depends on how comfortable you BOTH are with the process.
I'm not comfortable selling a deadly weapon to a complete stranger unless the transfer is legal and properly documented TO MY SATISFACTION, not just to what the law states. If I ever find a detective at my door asking why my pistol was found at a crime scene, I have no reservations or difficulty providing documentation of a legal and properly handled transaction.
Saying you are not responsible for what the buyer does with the gun is true. But you are responsible to make sure the person you are selling the gun to meets the legal requirements for purchase insofar as you are able. Until the gov't requires background checks for private sales, it's up to the sellers to keep the guns from criminals. If anything about the buyer makes me suspicious, I would call off the sell. But maybe that's just me?