That comment was made out of ignorance. Antitrust laws are by no means limited to sellers only.

Interesting, thanks.

"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."

I know this was pointed at grellas, but I think this is a misunderstanding when we look at price fixing and collusion. The illegality of collusion is secretly forming agreements to benefit competitors at the expense of other parties. Words like defraud can succinctly help you understand whether it is illegal or not when looking at these agreements.