First, get their Drivers License and make sure they are over 18 and reside in VA. Below it is suggested to give a Drivers License Num.
Second, make sure that you are not a licensed firearms dealer. Federal law restricts licensed dealers to sell handgun ammunition to people under 21. There are no state laws restricting the sale of any type of ammunition.
Third, make a contract saying how much ammo, what kind of ammo, price, and he agrees he is not a felon and over the age of 18.
Fourth, MAKE HIM SIGN IT!!!
Pretty simple. Hope this helps.
EDIT: did a few fixes.