Is selling permissible goods to immoral people considered aiding in sin? Is it permissible to sell to someone suspected of having unlawful wealth? And is it permissible to work for an entity whose source of funds is unknown?

If you are certain or it is highly probable that the materials you sell to a profligate person will be used by them to disobey Allah, then it is not permissible for you to sell them to him. The default assumption for the wealth of Muslims is that it was acquired through lawful means, and a Muslim should not harbor ill suspicions about their brethren. A mere doubt about another's wealth does not establish a ruling, because rulings are established by certainty or high probability.