The Civil War was a complex conflict stemming from myriad causes including slavery, trade, and federal structure, and as such it is a matter of opinion as to whether or not it was preventable. Many forces were in play at the war's outset and at the time many outcomes seemed possible.
Efforts to end slavery were well underway during the events that led up to the Civil War. Slave rebellions had spread like wildfire around the slave-owning Western world while abolitionists crusaded against its continuation and freedom fighters like Harriet Tubman struggled to ferry slaves to safety along the Underground Railroad.
Determining whether or not a war was preventable is a practical impossibility. Speculation is the only tool available, no matter how informed. That there was considerable resistance to the war is undeniable, but equally undeniable is that the season of war prevailed and the fight was eventually waged.
Had a non-violent solution to the problem of slavery's gross injustices succeeded in freeing the slaves, war might have been delayed or prevented. The North and the South might also have avoided combat if President Lincoln had been removed from power, or if he had not been elected, though what course history would have taken otherwise is impossible to say.