You would be hard pressed to find any positive outcomes concerning the slave trade in Africa. The western portion of the continent was ravaged by Europeans eager to find a labor force that would assist them in acquiring more wealth and resources. The effects were far-reaching and devastating.
The male population of Africa was hit the hardest. Statistics claim that two-thirds of the slaves brought to the New World were male. This forced removal of such a large number of males created a demographic disaster. The population of Africa would remain stagnant until the 1800s as a result of this forced migration.
Without the expected growth and advance of civilization, the African continent was devoid of progress in areas such as technology and medicine throughout the slave era. Many cultural traditions and norms were no longer sustainable and the continent fell into a period of stagnation, if not outright regression.
Slavery was big business for Europeans and Africans alike. It is estimated as many as 8 million slaves remained in Africa to serve at the pleasure of African slave masters. As rival groups sought to control areas in which to capture slaves, conflict ensued which further aided in the destruction of any cultural progress on the continent.
Without advancements militarily and with a significant loss of the male population, the continent of Africa lay open to the colonizing forces of Europe. Those forces would later pillage much of the resources Africa harbored, further setting the continent back in its quest to modernize and remain independent.
As for positive outcomes, that is a very difficult thing to find. Obviously some of the more powerful African tribes became even more powerful through the slave trade, as Europeans were often willing to pay a high price for captured Africans. It is difficult for me to say that one group benefiting at the expense of another is a positive, however. Without question the African Slave Trade was evil and served to keep the continent several steps behind Europe technologically, socially and militarily.