At key points along the coast, the European nations had made treaties with the local rulers allowing them to set up trading stations and slave factories.

At this point, the European traders entered into hard bargaining sessions with the representatives of the local ruler in which the manufactured goods from Europe, especially guns, were traded for African slaves.

When the deal was completed, the slaves were loaded on the ship, and the captain set sail for the New World.
Upon arrival in the West Indies, another bargaining process was begun.
Here the slaves were traded for local agricultural products which were wanted in Europe.

Then the ships were loaded with tobacco, sugar, and other West Indian produce and returned to Europe for still another sale and another profit.

At every point along the route, large sums of money were made.