What made it illegal was the immigration status of the person who instigated the sale and who now walks free.
Not true. It's a subtle but important point that keeps getting overlooked: it is the alleged knowledge
of the seller, or the fact that he had sufficient reason to believe
that the alien was illegal, that makes the sale illegal (for the seller).
I keep seeing the question of whether the seller "knew it was an illegal sale" - the fact is that it's only an illegal act for him if he knows
or has reasonable cause to believe
(whatever that means) that the recipient is disqualified. If he is justifiably ignorant of the immigration status of the recipient then he is innocent on that particular count. It is irrelevant whether or not he knew or suspected that the gun might be transferred to someone else.