And what reason would the southern slave owners have had to sell? What would they replace the slaves with?
Sure buying the slaves, if feasible, would have been cheaper than the war. You can say that about any war. But the wars come.
I read about the same situation in Russia in 1861 to 1866. There was a lot of unrest. The Tsar issued an Edict of Emancipation to free the serfs in 1859 and there was some political stuff and freedom for the serfs finally started 1861. The nobility had borrowed against the value of the serfs and the properties so that they were weakened and less powerful. The nobility were under the power of the Czar but they no ability to negotiate. the Czar Alexander the second was able to implement the rule. The Nobility were paid in government bonds. Civil War was avoided for 50 years. And they could of worked on the solution more over time. Alexander II was assassinated later.