There were so many reasons for a conflict that you must have heard: the immense profitability of slavery for the Southern planters and the lack of any restraint of it in the constitution, the threat that such a giant pool of unpaid labor posed for the economy of the North and its non-owned labor force, the moral indignation of the Abolitionists vs the moral intransigence of the Fire Eaters, the culture of violence and honor in the South, the threat a greater Northern economy and population posed to the political power of the South, if it could not expand slavery into the new territories. And, the simple impossibility of two different sets of laws for one country. It really is rather impressive it did not happen earlier. If the Missouri Compromise had not happened, it likely would have.

I am not sure what counterfactual scenarios have been proposed that could have avoided the war completely, but I can think of a couple of things that might have delayed it- if the Dred Scott decision had decreed that slavery could not exist in free states, instead of decreeing that slavery could exist anywhere, there would have not been the Fugitive Slave Law, John Brown might not have gone as far as he did, and the temperature might not have gotten so hot so quickly ...or if Buchanan had had the political genius of Henry Clay, he might have been able to patch some sort of compromise together in the last months of his presidency.

One interesting hypothetical that's been proposed is the cotton market: it had started being planted at least in India, and Indian cotton , if it had come on the market earlier, would have freed British mills from total dependence on Southern cotton, and perhaps caused a collapse of the Southern economy ( though the idea of a stagnating cotton market in the 1860's has been questioned ). What the South would have been able to do with a huge population of slaves and nothing profitable for them to grow is hard to imagine, but it would at least have dropped the value and so the price of slaves, and it would at least have been conceivable the US government could have bought them as a way of eliminating slavery.

One major argument is that the slaves could have been bought at a lower cost than the war.

Even today, that argument continues.

http://www.theatlantic.com/national/archive/2013/06/no-lincoln-could-not-have-bought-the-slaves/277073/

http://www.politifact.com/punditfact/statements/2014/mar/18/jon-stewart/jon-stewart-lincoln-tried-buy-slaves-free-them/

The basic math is that the war cost the north about $3.4 billion in 1860 dollars. Add in the cost to the South and it goes beyond $6 billion.

https://www.princeton.edu/rpds/papers/Golding_Lewis_Economic_Cost_of_American_Civil_War_JEH1975.pdf

There were roughly 4 million slaves at the time and the average price across gender and age in 1860 was roughly $800. Multiply the two and the cost to buy all the slaves is roughly $3.2 billion.

Anyone could argue those numbers are off a few billion dollars but if you add in the human suffering and loss of life, it would have been a much better solution.

Lincoln did make all sorts of offers and proposals to buy slaves rather than emancipate them for free but A) it was too late as the war started before he took office and B) no offers by slave owners were accepted.

There are a two major counter arguments as to why this didn't work:

1. Slavery was good and is worth fighting for - yes sadly, that was the main argument believed by the Confederates. Their most eloquent leaders on this principle were called The Fire Eaters and they wrote a lot on this topic and were very influential in moving the Southerners to chose war over Union. Thus, the war wasn't a rational financial decision where we can look at the math and say that makes better sense.
2. Economics - trying to buy up all of a product from millions of different owners isn't going to work. The law of supply and demand will allow owners to continue to raise prices. Plus, it was a massive cost to agree to up front with little public support to pay for it