Re: Entrapment?
Not sure this goes as far as entrapment as that would require the police to try to entice the suspect to buy the weapon rather than the suspect going looking for the weapon. Also he wasn't done for "buying a weapon" he was done for a conspiracy to import. You don't have to carry out the final act to be part of the conspiracy to commit that act.
He could have had a defence of saying that he was planning to write a book about the 'dark web' and knew that there were not any real goods available on the dark web and was setting out to buy what he knows would end up being a fake weapon. Therefore he set out to buy a fake weapon and that is what was delivered.