Can a for-profit business make use of unpaid volunteers?

It is highly unlikely. The organization generally would have to be nonprofit. When determining whether one is a volunteer, the controlling factor is the intent of both the business and the potential volunteer. Coercion must not be present.

If the person intends to volunteer his/her services in the spirit of devotion, self-sacrifice or service to the needy, that person is not an employee.

If the person is performing tasks that might otherwise be performed by an employee, it is not likely that such a person could be a volunteer. Because of the tacit threat of adverse action, employers and employees seldom deal with each other on an equal footing, creating