It's a double-edge sword, that would ultimately land the responsibility of the seller (FFL or not) to know the firearms laws of any or all the states. As rapidly as gun laws are changing, whether it be banned/prohibited guns, or magazines, or magazine capacities, keeping up with said changes would be a nightmare for the seller (FFL holders in particular), and could put them in an unenviable position.
I would think one would only need to follow the laws of ones own state, whether buying or selling.
Ultimately I would think it is the buyers responsibility to know and follow the law which is what private sales of firearms is now.
To whom may an unlicensed person transfer firearms under the GCA?
A person may sell a firearm to an unlicensed resident of his State, if he does not know or have reasonable cause to believe the person is prohibited from receiving or possessing firearms under Federal law. A person may loan or rent a firearm to a resident of any State for temporary use for lawful sporting purposes, if he does not know or have reasonable cause to believe the person is prohibited from receiving or possessing firearms under Federal law. A person may sell or transfer a firearm to a licensee in any State. However, a firearm other than a curio or relic may not be transferred interstate to a licensed collector
