The overwhelming majority of the women and men sold to and by Italians came from Eastern Europe and Central Asia. Merchants traded in Russians, Circassians, Tatars, Abkhazi, Mingrelli, Geti, Vlachs, Turkish, and others from the Balkan, Caucasus, and Central Asian regions, some of whom were Christians, captured by enterprising local traders or sold into slavery by debt-burdened parents. In late fourteenth-century Florence, most of the slaves were Tartars. Genoese traders sold Greek-speaking adherents of the Eastern Church in Italian and Aegean markets until the late fourteenth century, when the Genoese government no longer allowed it. Far fewer Greek slaves appear in Italian notarial sources after the turn of the fifteenth century, which suggests that the populations of Italian slave-owning societies now viewed the enslavement of Greeks to be as illegitimate as their own enslavement.
(...) The Genoese relied heavily on Russian, Circassian, and Tartar slaves into the 1460s. In Venice, Tartars stand out among the slaves sold there. Only the number of Russian slaves reaches nearly as high a figure. When they lost access to the Black Sea in the late fifteenth century, the Genoese and Venetians resorted to Bosnian, Serb, and Albanian captives of the Ottomans.15 Sub-Saharan African slaves begin to appear more frequently in Genoese and Venetian records in the second half of the century, at the same time that domestic slavery in Italy declined.
In colonies established by Italians, the origins of slaves changed more slowly than they did in Italy. The reluctance to enslave Greeks, new to the cities of Italy, did not extend to Venetian and Genoese colonies. In Venice’s colony of Crete, as in Frankish Cyprus and Genoese Chios, the enslavement of Greeks persisted into the fifteenth century, although, in the case of Crete, Greek slaves were imported to the island from elsewhere. The indigenous Greek-speaking Cretan peasantry occupied a social rank slightly higher than that of slaves, similarly tied to landed estates like serfs in Western Europe but without the feudal implications. In the port of Candia on Crete, Catalan and Venetian traders sold slaves they bought in the markets of Thebes, Naxos, and the emirates of Asia Minor, where Turkish merchants sold captives taken mainly along the Aegean and Anatolian coastlines. As the fifteenth century progressed, in the colonies, Balkan and African slaves gradually replaced slaves from the Greek Islands and Black Sea.
Surely the fact that Greece was mostly Ottoman by the early 15th century has something to do with the lack of Greek slaves around. Perhaps the Genoese government was giving them protected species status, of a sort. Not many Greeks were left by this time, and the Ottoman needed the manpower for themselves.
And so the greedy Italians ran out of Greeks, and resorted to bringing Africans instead. Talk about resource management.