The most prolific slave traders in this era were probably the Vikings, who abducted people from Germany, the British Isles and Southern Europe, and sold them as slaves in modern-day Turkey and Iraq. (This 1909 painting by Sergey Ivanov depicts people being sold to Arab traders by Vikings.)
The Mongols sold people from Eastern Europe as slaves in Mediterranean markets in return for manufactured goods. The Arabs too practised slave trading, taking people from sub-Saharan Africa and selling them in the markets of North Africa.
UNESCO.org section on the Slave Trade.
Encyclopaedia Britannica entries on Slavery and Slave Trade
United Nations Interregional Crime and Justice Research Institute section on Human Trafficking