Two-Sided Markets After Amex
How courts decide whether a platform is a two-sided transaction market under Ohio v. American Express — and where the framework has traveled outside payment networks, from airline booking systems to real estate listing networks, ride-hailing, and law-school admissions platforms.
Two-Sided Transaction Markets After Ohio v. Amex
Ohio v. American Express Co. supplies the vocabulary every later court uses: "a two-sided platform offers different products or services to two different groups who both depend on the platform to intermediate between them," and within that family, "[t]he key feature of transaction platforms is that they cannot make a sale to one side of the platform without simultaneously making a sale to the other." Because those platforms "facilitate a single, simultaneous transaction between participants," the relevant market must include both sides. Ohio v. American Express Co., 585 U.S. 529 (2018). The Court cabined its own rule, though: "it is not always necessary to consider both sides of a two-sided platform," and "[a] market should be treated as one sided when the impacts of indirect network effects and relative pricing in that market are minor" — newspapers selling advertising being the canonical example. Every post-Amex market-definition fight begins on that line between transaction platforms and everything else.
The Second Circuit was the first court of appeals to carry the holding outside payments, and it read the rule as mandatory: in a Sherman Act case involving a two-sided transaction platform, "the relevant market must always include both sides of the platform." Adopting the four-element distillation of Amex that Justice Breyer's dissent had set out — a business is a transaction platform if it "(1) offer[s] different products or services, (2) to different groups of customers, (3) whom the 'platform' connects, (4) in simultaneous transactions" — the court held that "the Sabre GDS is a transaction platform, and the relevant market for such a platform must as a matter of law include both sides." US Airways, Inc., for American v. Sabre Holdings Corporation, 938 F.3d 43 (2d Cir. 2019). Because the jury had been permitted to find a one-sided market limited to airline services, the verdict was vacated and the case remanded for a new trial on a two-sided market.
The Ninth Circuit then marked the framework's outer limits in a case about residential real estate listing networks. The pls.com, LLC v. Nar, 32 F.4th 824 (9th Cir. 2022) held that Amex is not a universal pleading hurdle: a plaintiff need not define a market for a per se claim or for a rule of reason claim built on proof of actual anticompetitive impact, so "Amex does not apply to these claims" — only "[f]or rule of reason claims based on indirect evidence . . . Amex may play a role." Nor could the court say in the abstract which platforms trigger the rule: "whether Amex applies depends on the facts," and the panel expressly declined to resolve whether simultaneous transactions or strong indirect network effects are the trigger. Even where the framework governs, "Amex does not require a plaintiff to allege harm to participants on both sides of the market"; it demands only an anticompetitive impact on the "market as a whole."
District courts have taken the framework well beyond payment networks. In ride-hailing, SC Innovations, Inc. v. Uber Technologies, Inc., No. 3:18-cv-07440 (N.D. Cal. May 1, 2020) let Sidecar's monopolization claim proceed because the complaint "addressed both sides of the market, as opposed to 'only one side of the platform in isolation,'" plausibly alleging that network effects let Uber "raise both passenger fares and commissions withheld from drivers without a rival increasing output to restore competitive equilibrium." And in RISNER v. LAW SCHOOL ADMISSION COUNCIL, INC., No. 2:25-cv-04461 (E.D. Pa. Apr 28, 2026), the court treated a law-school application platform as a two-sided transaction platform whose key feature is that it "cannot make a sale to one side of the platform without simultaneously making a sale to the other" — and dismissed a complaint trained on applicants alone, because "the mere fact that LSAC offers its product to the law schools for free does not show anticompetitive effect" when the "optimal price might require charging the side with more elastic demand at below-cost (or even negative price)."
Across these decisions, classification does the work. Where the defendant sells a single, simultaneous transaction jointly consumed by both groups — payment processing, airline bookings, an admissions clearinghouse — the market must include both sides, and "[e]vidence of a price increase on one side of a two-sided transaction platform cannot by itself demonstrate an anticompetitive exercise of market power." Ohio v. American Express Co., 585 U.S. 529 (2018). Where the platform merely exhibits network effects without transactional simultaneity, or the claim is per se or rests on direct evidence of harm, plaintiffs retain room to plead a one-sided market. The threshold question in any platform case is accordingly factual, and worth framing at the outset: what exactly does the defendant sell, and is it sold to both sides at once?