For years, activists, lawmakers, lobbying groups, think tanks and most Americans have agreed something should be done about giant tech companies’ power. With minor exceptions, no one has figured out how to do it.

Now, U.S. competition regulators at the Federal Trade Commission are getting creative. They’re zeroing in on an issue that has been less prominent in the past: how Big Tech dominance harms not consumers, but the businesses that sell goods and services on those tech platforms.

…

But the FTC has faced a puzzle. The ills typically associated with weak competition—such as higher consumer prices and inferior consumer choices—are difficult to prove in today’s tech sector. Regulators might argue that such harms are inevitable with companies accumulating so much market power, but how can they demonstrate that the damage from not intervening now is more than hypothetical?

That’s where monopsony enters the discussion. This focus is one of the newest ways the FTC is attempting to establish harms to competition from big tech companies, says Krista Brown, a senior analyst at the American Economic Liberties Project, a liberal think tank with which Ms. Khan has collaborated in the past. Marketplaces where companies are both a referee and a player, setting the terms of how the market works and also participating directly by selling their own goods and services, are of special concern, she adds.