Nice try, but the sale of a gun is not the same as sale of illegal drugs. The FBI agent suggested an illegal buyer and turned an innocent sale into a crime. If the agent hadn't suggested the illegal buyer, the kid might have accidentally sold the gun legally, and we can't have that, now, can we?
By your argument, any cop offering to buy drugs from a drug dealer would be entrapment.
Nice try, but the sale of a gun is not the same as sale of illegal drugs. The FBI agent suggested an illegal buyer and turned an innocent sale into a crime. If the agent hadn't suggested the illegal buyer, the kid might have accidentally sold the gun legally, and we can't have that, now, can we?