One of the largest of the pre-African-slave-trade groups of white slaves were the Ukrainian and Caucasian slaves traded to the Ottoman Empire. At its height, this business extended to North Africa, and was run as their major business enterprise by the Knights of St John, based at Rhodes until the Ottomans threw them out in 1522. So the upstream end of the enterprise was Christians selling pagans and fellow-Christians to primarily Muslim buyers.

One interesting angle was that Islamic law forbade castration, but didn't restrict the importation of eunuchs. So the supply route was deliberately taken outside Muslim territory so that slaves could be castrated to meet the demand for eunuchs.