"
Don't most states require that both parties be same-state? "
That usually applies only to sales
by FFL holders. And it applies to handguns but not rifles (except a few listed "controlled" rifles).
In most states, a private individual (not holding an FFL) can sell any firearm he/she owns to any other private individual (not holding an FFL) no matter where they are from and no matter the gun type.
At gun shows there will be some exhibitors who hold FFLs and they will tell you they can sell only to residents of the state in which they (the exhibitors)reside. But you will also come across exhibitors who do
not hold an FFL and are there to sell
their private guns - and they can sell to anyone regardless of where they are from.
Have run into this in Ohio, Kentucky, Tennessee, Georgia and Florida. In Ohio, I was not even allowed to bid on guns at a farm auction (conducted by a licensed auctioneer) because I had not yet gotten an Ohio Driver's License.
But guns owned in private collections and sold by the owner are private property, which is treated differently from the guns
bought for the purpose of resale by a dealer with an FFL.
Still, always best to double-check the laws of the state where one is thinking of buying a firearm.