I hate being wrong
You do not need a guilty mind to be criminally liable. The law is a strict liability. You broke the law because you sold it to someone that does not reside in your state. That is all the US Attorney would have to prove.
In a crime of strict liability (criminal) or absolute liability, a person could be guilty even if there was no intention to commit a crime
Reading the condensed version of a statute is not what one should do. I was incorrect. It is NOT a strict liability issue.
It is illegal 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;
The "knows or has reasonble cause to believe" makes all the difference. I read a condensed version and I should have known better!