Marseille is internationally known for his Soap, called Savon de Marseille, a heird from the time when Marseille was called the "Orient Gate", a maritime bridge between Europe and Middle East and Aleppo. This factory/shop is the oldest in Marseille, keeping this "soap tradition" maked with olive oil.
There are many shops "La Licorne" (owned by the original brand) near the Vieux-Port but you can visit the factory in the artistic district ( Cours Julien). You will love the diversity and the smells of the products as well as the traditional atmosphere there. They organise visit where you can follow all the processus of soap manufacturing.
Be careful there are to much Soap brand and some make fake Marseille's soap. To know if it's true one, put the piece of soap in water, it will float