Is selling something permissible to people of depravity and sin considered aiding in sin, and is it permissible to sell to someone whose money is known to be illicit or is suspected to be so, and is it permissible to work for an entity whose source of funds is unknown?

If you predominantly believe that the buyer will use what he buys in an act of disobedience to Allah, then it is not permissible for you to sell it to him, as a preventative measure (sadd al-dhara'i'). The fundamental principle regarding the wealth of Muslims is that it is lawful, and it is permissible to transact with them in what is permissible. It is not permissible for a Muslim to harbor ill suspicions about his Muslim brothers without certainty or a predominant belief.