As long as you are reasonably unaware of any disqualifying conditions (felon, domestic violence conviction, insane,etc) and you are both residents of GA, you may legally sell any legal long gun or handgun to him/her.
There are obvious exceptions for legal, but controlled items like Short Barrel Rifles, supressors, machineguns, and the like. Those still need the ATF tax stamp process to be followed.
ps - US citizen is not a requirement. Resident Aliens with proper documentation can also be eligible to purchase.