The statement that is not true about the Portuguese slave trade is:
It began with Portuguese merchants buying slaves from the king of Benin.
While the Portuguese did engage in trading with various African kingdoms, including Benin, the larger-scale involvement in slavery and the slave trade was not initiated exclusively through direct purchases from the king of Benin. The Portuguese initially engaged in capturing and trading slaves and later established relationships with various African leaders and kingdoms, leading to a more extensive trade network.