Firms must choose one role or the other:
“When you have \[the firm\] simultaneously serving as a seller and a buyer and running an exchange, that gives them an unfair, undue advantage in the marketplace, one that doesn’t necessarily reflect the value they are providing,” Senator Mike Lee (R-Utah) told the Journal in an interview. “When a company can wear all these hats simultaneously, it can engage in conduct that harms everyone.”
They just happen to be talking about Google and it’s Ad marketplace. But it kinda makes me wonder…what other situations could this describe?