If you had a roomful of people who wanted guns, and you knew that one of them planned to commit murder with that gun,
would you give them all guns anyway?
TRULY Stupid question....you would give everyone a gun EXCEPT the person you KNEW planned to commit murder.
Of course you don't just GIVE anyone a gun. Laws must be obeyed, background checks have to be completed, and a waiting period must be upheld. Of course you knew that. In your scenario, if you discover one wants to commit murder with the weapon of course you don't sell them the gun...and you report them to the police.
Next....