Is the money the husband earned from selling blank guns permissible, and is selling something he doesn't possess considered a permissible form of brokerage?

The sale of weapons is not forbidden unless it is known that the buyer intends it for an unlawful purpose. If it is for self-defense, there is no harm in it, and the sin of misuse falls upon the buyer. As for brokering, it is permissible if the commission is known, whether it is taken from one party or from both parties with their knowledge. If the husband acts as a broker for the seller and agrees with him on a commission, there is no harm in him taking it, because he is an agent for the seller. However, if he is not a broker and buys the weapon then sells it at a higher price, this falls under selling what one does not own, and the increase reverts to the original owner, unless there is another condition in the agency contract, citing the Hadith of Urwa al-Bariqi.