The key word is that a plumber "can" exploit one customer at a time. A plumber can exploit a customer by charging an exorbitantly high rate for work. They can

The key word is that a plumber "can" exploit one customer at a time. A plumber can exploit a customer by charging an exorbitantly high rate for work. They can choose to exploit their position of power as a supplier of services where a customer has an urgent need. Exploitation is not charging a high rate for work. It is charging a higher rate when a customer is not in a position to shop around.