Part of the question becomes "How popular is slavery in this world"? Not just allowed... does the average person like and accept slavery as a normal part of life? Or is there a significant abolitionist movement?

For example, compare popular perceptions of the rebellion of Spartacus and John Brown's raid on Harper's Ferry. The rebellion of Spartacus was pretty universally (amongst the free population) deplored, because slavery was viewed as right and normal; every Roman either owned a slave or wanted to own a slave (in fact, a sign of abject poverty was being unable to own a single slave). The Harper's Ferry raid, however, was viewed more ambiguously... while it was pretty universally deplored in the South, some people in the North applauded it, some deplored it... either because they thought of slavery as a right in the South, or because they disliked the methods used.

Your job, as a DM, is to determine reactions to this. How much are the average people going to get up in arms about this? Is there going to be covert aid to the rebels? If so, how are the rebels going to control their people? If the common folk are on your side, you have to be exceptionally careful to avoid making them mad, and any uncalled-for atrocities will do that. You also have to question who makes money from the slave trade and how they're going to respond.

A good set of books to read is the Guardians of the Flame series by Joel Rosenberg (not Joel C. Rosenberg, who writes political thrillers), specifically the first four or five. "The Sleeping Dragon" is arguably the best of the series, but for your purposes, you're probably going to want to look at "The Sword and Chain","The Silver Crown" and "The Heir Apparent". All of them deal with a group that decides to take on slavery by making slave-trading too expensive, and by providing technological incentives to stop slavery.