It is not permissible for a seller to touch the hand of a woman who is buying from him
Question

                There is a religiously-committed businessman who has a lot of customers, many of whom are women. When giving and taking his hand sometimes touches theirs, and that happens often. What should he do?.
Praise be to Allah.It is not permissible for a man to touch the 
    hand of a woman who is not permissible for him, because of the fitnah (temptation) involved in that. You have to avoid this and repent to Allaah 
    from that. You can buy and sell with woman by words only. You have to fear Allaah and avoid that which leads to fitnah. 
And Allaah is the Source of strength.