"Collusion, price fixing, and dividing markets is only illegal on the selling side."

IANAL, but didn't Standard Oil got broken up largely for being a monopsony? In fact the first two complaints from the DoJ were about sell-side issues:

"Rebates, preferences, and other discriminatory practices in favor of the combination by railroad companies; restraint and monopolization by control of pipe lines, and unfair practices against competing pipe lines; contracts with competitors in restraint of trade; unfair methods of competition, such as local price cutting at the points where necessary to suppress competition; [and] espionage of the business of competitors, the operation of bogus independent companies, and payment of rebates on oil, with the like intent."