Long before nations gathered to develop the concept of "universal human rights," treating outsiders with far less regard than your own countrymen was standard practice. Slavery in medieval Europe was a natural extension of the Roman Empire's policy of conquering new lands and taking their inhabitants for purposes of labor and entertainment. Slavery was built into the legal system: a slave's worth, what was allowable for slaves, how they could be traded or freed, etc. Wales in the 10th century had laws set down by Hywel Dda (he was mentioned back in January), and the Visigoths used slavery for criminals who could not pay fines.
One "softening" of the slave trade was promoted by the Roman Catholic Church, who worked to prevent slavery of "co-religionists." St. Patrick, who had been a slave, argues in a letter to British chieftain Coroticus against making captured people slaves because sinners are already slaves to the devil:
I am at a loss to know whether to weep more for those they killed or those that are captured: or indeed for these men themselves whom the devil has taken fast for his slaves. In truth, they will bind themselves alongside him in the pains of the everlasting pit: for "he who sins is a slave already" and is to be called "son of the devil." [source]
About 10% of the population of England were slaves at the time of the Domesday Book, although the word used to denote a slave, servus, was also used for those who we now know to be serfs.
Because of the church's opposition to selling Christians to non-Christians, other sources were sought. For Venice, this meant capturing Slavs and eastern European pagans to sell to Muslims. Caravans of slaves would be led through Austria to reach Venice. A document that surfaced in 1250 in Bavaria records the tolls paid in the opening years of the 10th century for crossing the Danube; they include salt, weapons, wax, horses, and more, including slaves on their way from Bohemia and the Kievan Rus to Italy.
Genoa and Venice both sold slaves around the Black Sea starting in the 13th century, selling to Muslims those from Baltic, Slavic, Armenian, Georgian, and Turkish lands. Genoa did a lot of business from Crimea to the Mamluks in Egypt who were originally slaves themselves. Amalfi, on the southwestern coast of Italy, was a major exporter of slaves to North Africa.
This focus on Italian cities is unfair, since so many other countries bought and sold slaves. This topic will go on for another couple posts, at least. Stay tuned.