The decision to abolish slave trading was found in Dubrovnik’s old statutes. Slavery was a legally recognised system in which people were legally considered the property or chattel of another. A slave had few rights and could be bought or sold and made to work for the owner without any choice or pay.
The system was allowed in most parts of Europe right up until the start of the 19th century but was first abolished in Venice in 960. The Croatian island of Korčula banned it in 1214 says Wikipedia.