Is it permissible to deal with a customer who sells cigarettes, for him to be an intermediary in other transactions in which his money is not involved?

If the transaction with the cigarette vendor does not involve the sale of cigarettes, then there is no harm, such as if you supply him with goods or he mediates for you in their supply without you purchasing from him. As for purchasing goods from him, if he acquired them with unlawful money, then buying from him is forbidden. If this is unknown, but most of his money is unlawful, then dealing with him is disliked.