A class-action court ruling on Halloween, stemming from an antitrust trial in Kansas City, Mo., is the talk of the town among real estate professionals here. A federal jury found that the National Association of Realtors and multiple large brokerage firms had “conspired to artificially inflate the commissions paid to real estate agents,” The New York Times reported that day, calling it “a decision that could radically alter the home-buying process in the United States.”