For several years NAR has been fending off accusations by US antitrust officials and private litigants that it has conspired to keep home-sale costs high in the face of major technological upheavals. This verdict is by far the group’s biggest setback yet. An NAR spokesman said, “This matter is not close to being final as we will appeal the jury’s verdict.”

HomeServices of America, a subsidiary of Warren Buffett’s Berkshire Hathaway, said it intends to appeal. “Today’s decision means that buyers will face even more obstacles in an already challenging real estate market and sellers will have a harder time realizing the value of their homes,” a company spokeswoman said. Keller Williams said it is considering an appeal.

Under the current system, sellers pay their own agent a commission — typically 5% to 6% of a home’s selling price — which is in turn shared with the buyer’s agent. Over the course of the trial, plaintiffs’ attorneys argued this model has suppressed competition by making it difficult for buyers and sellers to negotiate for lower rates.

“NAR and corporate real-estate companies have had a stranglehold on real-estate commissions for too long,” plaintiffs’ lawyer Michael Ketchmark said outside of the courtroom.

The news sent real-estate brokerage stocks tumbling: Redfin and Zillow both plunged as much as 10% before recovering some losses. Traditiona broker Re/Max was down 3%.