Is it permissible for me to market cars displayed in the showroom of a person whose money is illicit due to fraud, knowing that I do not wish to market his company but rather the cars offered for sale?

If the owner of the showroom is involved in fraud and his money is illicit, then it is impermissible to deal with him in marketing cars, because they are from his ill-gotten wealth, and marketing them constitutes cooperation in sin and transgression, as Allah Almighty says: "And do not cooperate in sin and aggression." Sheikh al-Islam Ibn Taymiyyah mentioned that usurped money or money taken through an unlawful contract must be avoided. Therefore, if it is known that a person's money is unlawful, then dealing with him and working with him must be avoided absolutely.