I am not an expert on this but you can do more reading on the relationship between Benin and Portugal. The Dahomey kingdom sold more than a hundred thousand war slaves in the 1780s. The capital city of Benin, Porto Novo, was first constructed to facilitate slave trade with the Portugese. In fact, the GDP growth of the Dahomey Kingdom in the 17th century was on par with most European kingdoms, only slightly inferior to that of Great Britain - as a result of the slave trade.

The slave trade itself resulted because of war. War slaves were, as a palace protocol, required to be killed, but the kings then saw the profit in selling them instead - a portion of the war slaves were still killed to comply with palace protocol. So, the pattern emerges: war among the African tribal kingdoms occurred, some people became war slaves, some of them got sold to the Europeans, who paid enough for the tribal leaders to wage more wars. It didn't help that West Africa was particularly fragmented during that time.

As for "Europeans just coming and stealing people away", you have to realize slave traders, like any other business owners, looked for a stable source of income. Simply going around West Africa looking for people to kidnap was not only incredibly dangerous, but also incredibly inefficient as a method to make money. Remember: in the 1780s, Benin could sell, more or less, ten thousand slaves per year. So why would slave traders go through all the trouble of kidnapping random African people when there was an open, booming slave market anyway? Not to mention that Benin was far from the only place in West Africa Europeans could buy slaves from.

Manning, Patrick: Slavery, Colonialism and Economic Growth in Dahomey, 1640-1960