"It is difficult to determine when slavery became an important economic factor in African history. Certainly African enslaved each other from an early date; this is a common feature of most societies pursuing an agriculture based on manual labor. By about 900 A.D., however, a regular slave-trade had developed between the Niger River valley and the Muslims of Spain. With Negroes brought from West Africa and Slavs from Russia, the Spanish Muslim capital of Cordoba became one of the greatest slave-markets in the world. With the decline of Muslim Spain, this bulk of this trade shifted to East Africa. By this time, some peoples of Africa had come to depend upon the slave trade, and Zanzibar had become the great slave emporium. Wars between African tribes were not fought to kill, but to take prisoners who could be exchanged with Arab slave-traders for imported goods. It has been estimated that 25% of the slaves taken out of Africa ended up in Muslim lands. Even more important, this centuries-old trade had rooted the institution in the African economy and had established the general pattern of that trade."
Zayde Antrim has also pointed out that African slavery was in existence well before the arrival of the Europeans.
"Not only was slavery an established institution in West Africa before European traders arrived, but Africans were also involved in a trans-Saharan trade in slaves along these routes. African rulers and merchants were thus able to tap into preexisting methods and networks of enslavement to supply European demand for slaves. Enslavement was most often a byproduct of local warfare, kidnapping, or the manipulation of religious and judicial institutions. Military, political, and religious authority within West Africa determined who controlled access to the Atlantic slave trade. And some African elites, such as those in the Dahomey and Ashanti empires, took advantage of this control and used it to their profit by enslaving and selling other Africans to European traders."
Hugh Thomas in his book on the history of Slavery, The Slave Trade, has detailed the African involvement in the production of victims for slavery. African monarchs often bought slaves from dealers, in order to sell them again to Europeans, to other Africans, or to Arabs especially. The rulers of Benin, the kings of Ashanti, Congo, and Dahomey; and the Vili rulers of Loango, sold great numbers of slaves over many generations. Jean Barbot, who was on a slave ship during the 17th century said "the slaves [whom the African monarchs] possess and sell are prisoners of war, or, if from among themselves, are condemned to slavery for some crime. But there are also those who have been kidnapped by their compatriots, these being mainly children...". The Muslims in Africa also had a heavy interest in the slave trade. Hugh Thomas noted that the Muslims traded their African slaves to many countries, selling them as far off as Java and India in the Middle Ages; and even to the Chinese.
"In Africa down to the 1930's, the various tribes continued to raid one another to capture slaves both for domestic use and to sell to outsiders. Moreover, in spite of the picture presented in Alex Haley's Roots, white slave traders almost never entered the interior in pursuit of prey but rather purchased their cargo from Africans at the ocean front; coastal Africans would not allow Europeans either into or through their own countries ...some scholars claimed that slavery in Africa was a response to the international slave trade, but it is now obvious that (Black) slavery was an old domestic institution that was adapted for supplying the international market when it developed." 
Hellie's view was echoed by Thomas Jackson in American Renaissance.
"Among the Tuareg of the southern Sahara, during the 19th century 70-90% of the population were probably slaves. In the Sahel and the savannah, half the population might be slaves, while in the forests the figure could be as low as 10 to 20 percent. Professor Oliver in "The African Experience" argues that the European and American demand for slaves may not have increased the supply. White slave traders almost never ventured into the interior and were dependent on a varying supply over which they had no control. They followed the flow of captives rather than create it, shifting their bases up and down the coast according to where tribal wars were producing the most slaves. Africa clung to slavery long after it was abolished elsewhere. Between the world wars, Liberia, founded by freed American slaves, was censured by the League of Nations for practising slavery." 
An article from the Mel Fisher Maritime Heritage Society in Florida shows how widespread slavery was at that time.
"At the dawn of the transatlantic trade, slavery was not new, nor were Africans the only people to be enslaved. Slavery is mentioned in the Bible, and most ancient societies including Egypt, China, India, Mexico, Peru and Greece made use of slave labor. Slaves were usually prisoners of war, conquered peoples, debtors or criminals. In Europe, the Roman Empire took slaves from every nation it overcame, including England, France, Spain and Germany. Slavery persisted in the Mediterranean Basin throughout the 17th century.
The Economist admitted that the British, French, Spanish and Portuguese were not the only people involved in the African slave trade.
"In Africa, slavery was accepted as the norm in most societies. Before Europeans arrived, and long after, millions of Africans were marched north across the desert by Arab traders. Most had been taken in war. The guns given in exchange helped wars to multiply and grow larger. Prisoners who might earlier have been absorbed into the victor's army or workforce, or killed, were now fed to European and American ships seeking human cargo, from Gambia round to Mozambique. Other Africans were sold as slaves because they owed a debt; some even by their own families. Some, like Equiano, were simply grabbed; though only in the early years by Europeans, because that upset relations with the African coastal kings, who wanted to keep control of the trade."