Everyone has a price.
Be it slaver or slave, you have encounter my character and now we're traveling together. Be it taking over someone else's plantation, or forcing your character along an uneasy path to profits. Note: Escaping or betrayal may trigger PVP scenarios.
Your character has personal issues with my character, or slavers in general. Perhaps this triggers or warrants a desire to bring my character to justice through your hand. Perhaps my character becomes a captive and is held for a number of days. Perhaps he is tortured for information on his stock or where he planned to take his next batch of slaves from.