... One of the most persistent myths about the slave trade is also one of the most pernicious: that Africans' role was wholly that of hapless pawns. Except in a few regions of the Congo river basin in the last decades of the 18th century, Africans themselves controlled the supply of slaves, selling them to Westerners in the number they choose at the prices they negotiated as equals. To be sure, whites tried to play off slavers against each other to get lower prices; but African slavers played off Western buyers against each other too in order to drive the prices back up. Even as late as the mid-19th century, few Africans viewed slavery as an institution that needed to be explained, still less as an evil to be decried. Slavery was part of the furtniture of every day life; depriving others of their liberty wasn't morally problematic, though it was bad to enslave the wrong person. Muslims nominally weren't allowed to enslave fellow Muslims, though in practice breaking this rule was often permitted. Yet a Western reader must understand that less than century prior to this period, the Europeans were just as interested in the slave trade as their African counterparts, and there are more than enough examples of Europeans being sold into slavery, whether chattel or debt-bondage, to ignore retorts that Westerners' culture and their moral foundations were somehow superior to that of Africans. Africans sold their fellows into slavery for longer than the Western powers, with the glaring example of the United States, less because of their different attitudes towards liberty than because of their different economic systems.
Very broadly speaking, slaves were the only form of private, revenue-producing property recognized in traditional African law. Where as in Europe, the most important form of property was land, and the aristocracy consisted mainly of large landowners who could buy or sell property with little legal restriction. In Western and Central Africa along the Atlantic coast by contrast land was effectively owned by the state - sometimes personally by the king, sometimes by kinship or religious groupings, often by the government itself - with the sovereign exercising supreme authority in the manner of a corporation's chief executive. No matter what the exact arrangement which held true in any given polity, and though there were exceptions, for the most part land could not be bought, sold, or readily taxed. Which could be sold and taxed though was labor. African leaders who wanted to enrich themselves and increase their power didn't think in terms of occupying territory but instead of controlling populations. Napoleon sent his army to seize the nations of Europe; an African Napoleon would have sent his army to seize the peoples of Europe.
As was the case in most of Europe prior to the rise of abolitionism, Africans could be punished to a sentence slavery if they forfeited their membership in society by committing a heinous crime. People could be enslaved, as in Europe prior to the 1848 Revolutions, too, to repay a debt, whether incurred by themselves, their families, or their lineages. In times of drought or flood they pawned family members to other members of their extended clans. Sometimes they would sell themselves. But the most common way to acquire slaves was by sending troops out to gather them - that is, by war. 19th century West Africa was even more politically fragemented than Europe. A map from the period clearly shows at least sixty different states of wildly varying size, with an equal amount of incorporated peoples and tribes outside of any centralized control. When leaders in one empire wanted to aggrandize their status, a border with 'others' was always nearby. In the beginning of the Transatlantic Slave Trade, when European ships first became a constant presence on African shores, the difference between the two systems was more a matter of culture than of economics. Europeans could buy and sell labor just as the Africans could - that was the purpose, to cite one example, of indentured-laborers. And Africans could effectively own land by controlling the population of the peoples who used that land. In both cases the owners would profit; the difference merely lay in the route to these profits.
However Europe underwent the revolutionary transformation of the Napoleonic Age, an shift in epochs that only touched the edges of Africa (e.g. Egypt) in the same time period, which gradually but steadily led to the differentiation between the two systems. One big distinction is that labor can be taken from one place to another in a way that land cannot. Because labor was the main form of property, rich Atlantic Africans almost by definition owned a lot of slaves (1). European-style plantations were rare in this part of the world, so big groups of slaves rarely were found working in the fields as was common with slaves that were imported to the Americas. In addition, much slave labor consisted of occassional or seasonal tax or tribute labor to the sovereign. Instead slaves were servants, miners, workers, or more commonly, soldiers...
... Often urban slaves in Africa did almost no work, as wealthy, powerful slave owners kept more slaves than they needed, in the same way wealthy, powerful Westeners would pile up rights on unused land. Slaves were effectively adornments to their owners. Western observers noticed the surplus, and how tribute slaves weren't always employed as laborers in the field, and they often conclucded that African slavery was inherently less brutal than slavery in the Americas. In pure terms of long-term survival, this seems to have been true. On a tobacco planation in North America, slaves who couldn't work were inherently without value, and were treated that way. By contrast in African empires even the oldest, more infirm would wear fine clothes and jewelry as a means of showcasing his owners power and wealth. Or they could simply be interesting to their owners. For several years Agadja, the first king of Dahomey, kept an utterly useless palace slave who had been seized as a debt payment: a hapless Briton named Bulfinch Lambe whom the monarch simply enjoyed talking to. African slaves were also more likely to be granted liberty after a period of service than their American counterparts, both because captives often had some sort of kin, religious, or ethnic connection to their captors, and because as subject they were valueable to their respective empires. By contrast a freed plantation slave was a total loss to his owner if they were still capable of work. These factors mitigated the callousness of the institution. Still, one suspects the Africans wrested from their homes in war raids would not have celebrated the humanity of the system...
(1) Atlantic Africa ITTL refers to those regions of Africa which border that ocean between the Sahara and Namib deserts, an area which is mostly tropical or sub-tropical in climate with poor soil compositions. Contrast this to the subtly more European-like coasts of Eastern Africa between the Cape of Good Hope and the tip of the Horn of Africa.