Maybe a little paranoia here. As long as you are selling within the state and both you and the buyer are residents of the state, state law applies. However, I would not sell a handgun to anyone (except a federal licensee) without going through a dealer (as I must in MD in any case), so it goes through the instant check system, which is not available to individuals or holders of collector licenses.
When I have (rarely) sold guns, I get the buyer to fill out a 4473 form (I have some). With an individual sale, it has no legal backing, but if a buyer won't fill it out or can't provide ID that shows he is a resident of the state we are in and the state of which I am a resident, I won't sell him a gun.
Maybe it is paranoia, but the problem is that at some point, you were down on paper as the owner of the gun. If it ends up in the wrong hands and something bad happens, a whole lot of people will be trying to trace it and figure out a way to hang some part of the responsibility on you, just to prove they are "doing something to stop gun violence."
Jim