Yes, but never (to my knowledge) to the extent of the slavery most westerners think of today.

For example the Ottomans preferred their janissaries to be white boys. As a result many young boys from the balkans were captured by groups like the Barbary pirates and sold.

Short answer; never like the African Slave trade but yes it wasn't completely foreign a concept.

Aristotle thought that certain people were naturally 'slavish' but i don't think he was talking about race. Unless your into etymological debates about the word Slavic.