Africans had been selling slaves to each other and to Arab traders for centuries. When the Europeans arrived, they became just another buyer that tapped into this resource. The Europeans brought goods that were either cheaper or unattainable in Africa and at first traded them for gold and other precious metals and stones. As time progressed and the need for bonded labor rose, the goods the Europeans brought were exchanged for slaves.

Edit: I forgot to mention that in many cases, slaves were one of the only forms of private property that Africans had. Land in many African kingdoms was owned by the state or the local lord and the only way to make money was to acquire slaves to work as much land as possible. Slaves were a valuable commodity and were one of the only items that could be exchanged for goods that the Europeans brought.