Why did the kingdom of benin eventually sell its own people into european slavery?

The kingdom of Benin is what is now known as Nigeria. It was Benin in the late 1400s and it began to expand their power by conquering neighboring people. They took many captives of war as their slaves. The Portuguese began to trade slaves from Benin for gold but eventually Benin ran out of slaves to sell. The Portuguese began looking to other kingdoms to purchase slaves, unwilling to loose out on the gold, the kingdom of Benin began enslaving their own people to sell.