The main thing that the slave trade did to African political structures was to give more power to those nations or kingdoms that were in position to profit from the trade.
In Africa, most of the slaves did not come from coastal areas. Instead, the slaves came from inland. They were captured and sold by the coastal people to the Europeans. This made the coastal nations much more powerful. By selling slaves to the Europeans, these nations got access to such things as trade goods and guns. These things gave them much more economic and military power and allowed them to become more politically dominant.