Volga Bulgarian slave trade
The Volga Bulgarian slave trade took place in the Volga Bulgar Emirate in Central Asia (in modern Eastern Russia). Volga Bulgaria was a buffer state between Europe and the Islamic world and played a major part in the trade between Europe and the Middle East during the Middle Ages from the 10th century onward, and slaves were one of the main goods.
Source: Wikipedia — Volga Bulgarian slave trade (CC BY-SA 4.0)