(If slavery was an issue, then the North should've just bought out the South - likely would've been much cheaper than the actual war.)

The North (well, congress) tried to buy out the South (well, slaveowners). The South rejected it. There were actually multiple attempts at this, some before the war, some during the war.

The thing is, the War between the States really truly was about slavery, nothing else. The dodge that it was about states' rights comes down to exactly one right -- the right to keep slaves. Compare with such travesties as the fugitive slave acts, which they pushed through congress, which actually did greatly infringe the rights of the northern states. The southern states, despite some of their propaganda, did not generally support the right of secession. Their Constititution explicitly forbade it. Every single article of secession passed by their state legislatures explicitly called out slavery as the reason for secession.

The odd thing is that slavery was not in any immediate danger. But with the election of Lincoln the southern states saw that their grip on the country was not as absolute as they desired, and they threw a tantrum, because they demanded not only the right to have slaves, but that the rest of the country not judge them for it.