If you are that concerned you need to take my other advise: Explain to the family that you feel you cannot ethically represent these firearms as legal, so you will not handle the transactions. This is not a breach of your word, because it is based on the belief that the firearms are illegal.
By the way, why are you dealing with shipping the firearms? The quickest way to LEGALLY sell arms is a table at a local gun show. Get a table, ask people to complete the form (for your records only), and verify their information against their ID. Done deal!