You can't sell a hangun to someone from another state, unless it is via an FFL transfer.
If you ship it out of state (you cannot use the USPS, but you can ship via FedEx or UPS) it must be shipped to an FFL in the state the person lives.
The person cannot travel to your state and purchase the handgun. Title 18, Sec. 922A (3) (5).
Sec. 922. - Unlawful acts
(a) It shall be unlawful -
(3) for any person, other than a licensed importer, licensed manufacturer, licensed dealer, or licensed collector to transport into or receive in the State where he resides (or if the person is a corporation or other business entity, the State where it maintains a place of business) any firearm purchased or otherwise obtained by such person outside that State, except that this paragraph (A) shall not preclude any person who lawfully acquires a firearm by bequest or intestate succession in a State other than his State of residence from transporting the firearm into or receiving it in that State, if it is lawful for such person to purchase or possess such firearm in that State,
(5) for any person (other than a licensed importer, licensed manufacturer, licensed dealer, or licensed collector) to transfer, sell, trade, give, transport, or deliver any firearm to any person (other than a licensed importer, licensed manufacturer, licensed dealer, or licensed collector) who the transferor knows or has reasonable cause to believe does not reside in (or if the person is a corporation or other business entity, does not maintain a place of business in) the State in which the transferor resides; except that this paragraph shall not apply to
(A) the transfer, transportation, or delivery of a firearm made to carry out a bequest of a firearm to, or an acquisition by intestate succession of a firearm by, a person who is permitted to acquire or possess a firearm under the laws of the State of his residence, and
(B)the loan or rental of a firearm to any person for temporary use for lawful sporting purposes;
Note, I left out all the paragraphs that did not pertain to the original question.
"To preserve liberty, it is essential that the whole body of people always possess arms..." (Richard Henry Lee)
"The best we can hope for concerning the people at large is that they be properly armed." (Alexander Hamilton)