Slavery reduces human beings to cattle. Sadly, this inhumane practice has existed through all times and cultures. Ancient Egypt, Greece and Rome, for instance, were slave societies. Economically speaking, slaves are 'produced' and traded. In this weird business, the West has a bad reputation, and well deserved too. But Europeans were neither the only nor the biggest slave traders. Actually, the opposite is true. They were the smallest traders, the last to make their appearance and the first to leave, abolishing the institution on the way out. That 'the West was the major slave trader' is one of the myths busted by Paul Bairoch, economic historian at the University of Geneva (Economics and World History, Myths and Paradoxes, Harvester Wheatsheaf 1993; pp. 146-147 here).
The Muslim contribution, on the other hand, is generally grossly underestimated, even by Africans. For black slaves, exported from Africa, here are the figures:
They are taken from African Economic History (1987, Currey and Heinemann 2003 reprint p. 275) by Ralph Austen, Professor of African History at the University of Chicago.
The Islamic total adds up to 17 million African slaves exported, the Atlantic (i.e. European) counterpart being 11.7 million. The figures for the Atlantic trade are confirmed by others and are more ore less universally accepted today. The Oriental trade is much less well documented. Bairoch's figure is somewhat lower ('14-15 million') but much bigger numbers have also been advanced. For 1400-1900, less than half of the Oriental trading period, economist Nathan Nunn gives an estimate of 5.5 millions (The Long-Term effects of Africa's Slave Trades, Table 2). All in all, there seems to be little reason to question Bairoch's qualitative conclusion that compared to the European slave trade, that conducted by the Islamic world started earlier, lasted longer, and involved a larger number of slaves (Bairoch p. 147). Muslims were slave traders for eight centuries longer than Europeans, and the illustration below (Bibliothèque Nationale de France, manuscrit arabe 5837, f° 105) shows a slave market in Zabid (Yemen) around AD 1230.
For black slaves, there is a third major trader, all too often unmentioned: the Africans.
Most of the recent studies, of whatever school, indicate that the Africans were not merely victims of slave trade, but also actors. (...) One can estimate that (...) the internal trade has resulted in reducing 14 million people into slavery.
(Olivier Pétré-Grenouilleau, Les traites négrières, Gallimard 2004, Folio Histoire 148, pp. 226-227. French original here. The estimate is due to Patrick Manning, Professor of World History at the University of Pittsburgh.)
Slavery, both 'mercantile' and 'domestic', was an institution solidly established in black Africa. (p. 93) Those offered on the marketplace resulted from razzias and wars, and occasionally from an adaptation of ancestral laws, as certain offences became punishable by reduction to slavery and deportation. (p. 91)
For an eyewitness, we turn to the Belgian army officer Camille Coquilhat, who served under Stanley and helped establishing the Congo Free State which existed from 1885 to 1908. He published an elaborate account of the events in which he took part, called On the Upper Congo (French Sur le Haut Congo, full text here). Slavery is mentioned on roughly 15% of the pages (73 out of 558). Here is sample.
We had been informed of the kindness of the native chief of Bonny; we paid him a visit. He received us under a veranda in front of his house and politely offered us rum and champagne. This respectable old man, who looked at least sixty years old, began to tell us about the good old days; how business prospered then! He mentioned ten thousand slaves he had sold to Europeans! Alas, all had changed since then. (p. 15, French original here.)
Many black slaves, though, did not leave Africa, and qualify under 'domestic' or 'internal' slavery. For their everyday life they may have been better off than the others. But theirs may have been a short one. They were routinely butchered in funeral rites, or bought merely to be eaten the next day (Coquilhat p. 269-271).
Summarizing, the figures mentioned above give rise to the following Hall of Shame of black slave traders (estimated order of magnitude in millions, not all equally reliable):
1. Muslims (17)
2. Africans (14)
3. Europeans (12)
If this quantification of misery makes you uncomfortable, feel free to reverse the order or to call it a draw. The conclusion won't change: these are huge crimes against humanity, and all are guilty.
Now for the 'production' of slaves. Europeans were not involved, but Arabs were, and not on a small scale. At a time when European slave trade had ended, Coquilhat met a party of 300 Arab slave hunters having devastated an area 'bigger than Ireland', collecting some 2300 women and boys and leaving the men dead (p. 393-395). The Muslim case is further aggravated by their three centuries of systematic white slave hunt in the Mediterranean. In Christian Slaves, Muslim Masters: white slavery in the Mediterranean, the Barbary Coast, and Italy, 1500-1800 (2003, Palgrave Macmillan paperback 2004), Robert C. Davis, professor of History at Ohio State University, writes
The result, then, is that between 1530 and 1780 there were almost certainly a million and quite possibly as many as a million and a quarter white, European Christians enslaved by the Muslims of the Barbary Coast. (...) In the sixteenth century especially, during which time the Atlantic slave runners still averaged only around 3,200 Africans annually, the corsairs of Algiers –and later Tunis and Tripoli– were regularly snatching that many or more white captives on a single raiding voyage to Sicily, the Balearics, or Valencia. (p. 23-24)
Even when compared to the atrocities of the Atlantic slave trade, which shipped some 10-12 million black Africans to the Americas over four centuries, these claims about what a hundred years of Mediterranean slavery accomplished do not dwindle into insignificance. Nor is this to deny or trivialize the well-documented enslavement of Moors and Turcs which was going on at the same time. Certainly, the Spanish, Tuscans and Maltese were all eager participants in the enslavement of their Muslim foes, largely to work them as galley slaves. Among Christian states, however, the practice was never as pervasive or as massive as in Barbary and died out sooner, as most European nations switched from galleys to sail and from slaves to convicts in those galleys they did retain. (...) Despite some attempts to label the two forms of Mediterranean slavery as pernicious mirror images, most students of the period still have to agree that, at least after 1571, corsair slaving was 'a prevalently Muslim phenomenon'. (p. 8-9)
To give an idea of the scale: christian slaves made up perhaps a quarter of the population of Algiers, a city whose very existence was predicated on corsair piracy and slave running. (p. 103) At some point, the market collapsed under its own weight, and in mid-sixteenth century Algiers a white slave could be traded for an onion. (p. 133)
Also in the white slave department: the Ottoman empire extracted considerable numbers of white slaves from the neighbouring regions of the Caucasus (Circassia and Georgia). Here too, the market collapsed and in 1856 the price for a 'good middling' Circassian girl (much in demand for their beauty) had tumbled from 100 to 5 pounds. (Read here.)