The U.S. Supreme Court refused Monday to review claims that Zillow and the National Association of Realtors blocked competition through an optional association rule that relegated a defunct brokerage platform's listings to a secondary tab on Zillow's site.

The justices denied the petition for review from Real Estate Exchange Inc., known as REX, in an order list without explanation, as is customary. The listing platform was trying to challenge a Ninth Circuit ruling that refused to revive its antitrust claims targeting changes Zillow made to its website to comply with the policies of NAR's local associations after joining them.

A representative for NAR said in a statement on Monday that the group has said from day one that the optional "no-commingling" was not antitrust violation.

"Both the district court and the 9th Circuit Court of Appeals affirmed this, and we are pleased their rulings will stand," the statement said. "Local MLSs play a key role in fostering transparent, competitive, and fair housing markets by delivering consumers the most accurate and up-to-date information on home listings. While the optional rule is no longer in effect, NAR remains committed to protecting the benefits MLSs provide agents, consumers, and the industry."

A representative for Zillow said in a statement on Monday the company is pleased the matter has reached the expected conclusion after the courts affirmed its position at every level

"Zillow was founded on increasing transparency in real estate and we have a long history of advocating for practices and launching products that do just that," the statement said. "This decision reinforces our long-standing commitment to transparency, innovation and prioritizing consumers."

The case centers on Zillow's adherence to a rule requiring properties from NAR-affiliated multiple listing services to be shown separately from other listings, which REX claims effectively destroyed its business by causing an 80% drop in views of its properties from Zillow.

U.S. District Judge Thomas S. Zilly tossed REX's antitrust claims against Zillow and the trade group in a 2023 summary judgment ruling ahead of trial in Washington federal court, after finding the optional rule could not be used as evidence of a conspiracy.

The judge said there was no concerted effort to make the website changes because NAR's local associations choose individually whether to adopt the rule.

REX's additional claims went to trial, where a Seattle jury ultimately found Zillow had not engaged in false advertising over the labeling of the separate tabs on its website. The jury also rejected consumer protection claims after finding Zillow had offered justifications for its design changes.

A Ninth Circuit panel refused in early March to overrule REX's summary judgment loss or the jury verdict against REX. The appeals court denied a rehearing bid in April.

The Supreme Court petition from REX focused on the summary judgment ruling and asked the justices to review whether a trade association can immunize a rule from antitrust scrutiny by making it optional for its members.

REX contended the trade group is controlled by its broker members who generally compete with one another to represent homebuyers and sellers. This, the petition argued, means NAR's decision to publish the rule was not the action of a single entity, but rather concerted action for antitrust purposes.

NAR's optional "no-comingling" rule is no longer in effect, but Zillow and the group have insisted the courts were right to find that the rule did not violate antitrust law.

REX is represented by Charles R. Flores of Flores Law PLLC and Bennett Rawicki of Hilgers Graben PLLC.

Zillow is represented by Jesse Beringer, Laura B. Najemy, Aaron Brecher and Aravind Swaminathan of Orrick Herrington & Sutcliffe LLP and John "Jay" Jurata Jr., Steven A. Engel, Russell P. Cohen, Brian Kulp, Lauren Schnepper Junior and Erica Fruiterman of Dechert LLP.

The National Association of Realtors is represented by Christopher G. Michel, Michael D. Bonanno and Michael J. Sebring of Quinn Emanuel Urquhart & Sullivan LLP.

The case is Real Estate Exchange Inc. v. Zillow Group Inc. et al., case number 25-326, in the Supreme Court of the United States.