Intra-state sales between two private parties is legal as long as there isn’t a state/local prohibition. For example, I could sell a handgun to another resident of my home state but they have to have a pistol purchase permit. Kalifornia (and most recently Kolorado) and several other states, for example, requires that all intra-state sales between two private parties go through an FFL dealer in that state.
Inter-state sales and transfers between two private parties is wholly ILLEGAL per federal law. The sale needs to go through an FFL dealer in the home state of the transferee. The person selling the firearm can transfer it to an FFL dealer (either in person or by shipping it to them) for subsequent transfer to the purchaser but is violating the law if they do a direct physical transfer to the purchaser who’s a resident of another state.
In Praise of Private Gun Sales
http://gunwatch.blogspot.com/2013/12/in-praise-of-private-gun-sales.html
Thank you for the perfect explanation. I will also mention that when I visit Ohio, I cannot purchase a weapon at a Gun Show without a valid in-state ID, e.g., resident of Ohio. This is a pain because guns are so cheap in Ohio compared to around Fort Bragg which is flush with GI Ca$h.