The cardinal sin under antitrust rules is a conspiracy to fix prices. Competition legislation throughout the world therefore universally sets out, as its first prohibition, a rule forbidding any agreement to fix (or otherwise coordinate) price levels. This is evidenced by Section 1 of the U.S. Sherman Act, Article 101 TFEU, and the opening salvos of countless pieces of national legislation worldwide.
This archetypal prohibition typically applies to suppliers conspiring to extract higher prices from consumers, and, indeed, this type of situation forms the basis for the bulk of enforcement activity. But the cardinal prohibition also applies to powerful buyers conspiring to extract sub-competitive prices from suppliers. Market power can take the form of monopoly or monopsony, and antitrust rules are agnostic: any distortion of free competition is equally worthy of investigation, and, if necessary, prohibition and penalization.
While the abuse of buyer power is relatively rare, it is no less anti-competitive than any cartel. It arises particularly in the case of labor markets, where powerful employers may conspire to extract sub-competitive wages from workers. Similarly, powerful commodity buyers might conspire to fix prices from fragmented suppliers of certain inputs. That said, there are, of course, also situations where buyers may legitimately coordinate to ensure that powerful suppliers do not extract supra-competitive prices and ultimately exploit con...