Real estate brokers generally don't take kindly to those in the business who sell services at a discount or who violate the confidentiality of house listing. Gina M. Williams, a maverick broker in Northridge, Calif., a Los Angeles suburb, has outraged and frightened the real estate establishment by doing both.

Williams' company, Supermarket of Homes Inc., charges a lower commission than other brokers in its area. Because of aggressive marketing, the company contends, it has been harassed and its employes have endured "a wave of terror" for the past two years.

Williams and her staff say they have received death threats and obscene phone calls from other brokers. Burglars broke into the office, ransacking furniture and files. Garbage was dumped into a client's yard; other clients had their "For Sale" signs smashed and axed. Last month, the word "death" was painted on Williams' home.

Real estate groups disclaim responsibility for the incidents, which are under investigation. But Supermarket of Homes is suing the local realty board, the California Association of Realtors, the National Association of Realtors and individual brokers both in state and federal courts.

The suits charge them with an illegal boycott asnd restraint of trade by limiting access to the multiple listing service, which provides confidential descriptions of properties available through its member real estate brokers.

The suits seek $10 million in damages and, more important, an order allowing home buyers and sellers to participate in the multiple listing service for a reasonable fee. In another move that has real estate brokers stewing, Williams has started selling copies of pages from the multiple listing service to the public while the courts try her antitrust complaints.

Many legal experts think Williams will lose her antitrust cases. Courts have been reluctant to find that multiple listing services violate the law as long as they are reasonably asccessible to real estate professionals. And requiring access to the lists by any home seller or buyer would be stretching the law considerably, says Norman Miller, an assistant professor of real estate at the University of Georgia.

But a California judge has refused so far to stop Williams from selling multiple listing information. And if a court decides that it is legal, the effect on real estate brokers could be devastating, industry professionals agree. Buyers would be able to find sellers without the help of brokers, and sellers could, in effect, use the listing services to advertise their homes without hiring brokers.

That prospect worries real estate groups, which have filed counterclaims against Williams charging that she violated her membership agreement with the local realty board to keep multiple listing information confidential and violated the board's copyright of listing-service books.

Williams' continued sale of multiple listing information would "completely alter the status quo" and would work the "further evil" of cutting out the brokers," the California Realtors' association asserts in court papers.

Williams' lawyer, David Barry, contends that 75 percent of home sellers would sell their own homes if they had access to the listings. The other 25 percent would seek discount services, he says, cutting residential real estate commissions as much as 90 percent.

Willialms' trouble began in July 1978, when she started charging a flat fee of $1,995 for selling a home. That's about 5,000 less than the 6 percent commission on the average home sale price of about $120,000 in the San Fernando Valley.

At first the 41-year-old agent was deluged with listings. "We couldn't write them fast enough," she said. But her clients soon became aware that other brokers weren't showing their homes. Indeed, brokers concede that they steer customers away from homes sold at discount fees, saying they can't afford to operate on discounted commissions. California's real estate commissoner, David H. Fox, asserts, however, that such steering is unethical and possibly illegal, because brokers are required to put clients' interests ahead of their own.

Williams says customers have deserted her in droves for regular brokers, who have been able to sell their homes quickly. She was forced to cut her sales staff from 17 to four, and her state court complaint says Supermarket of Homes" is about to go out of business."

But so angered are they by what has happened that Williams and her husband-partner Lawrence say they are determined to stay in business whatever the courts decide.

"We are going to jam it down the brokers' throat so hard they won't be able to move," Lawrence Williams says.

And Williams herself isn't prepared to return to charging standard commissions. "We couldn't go back becasue of what they've done to us," she says.

Sales of multiple listing pages, meanwhile, remain popular with home buyers and seller. Williams says she has sold thousands of copies at 10 cents a page. Peter Mailler, an electrician, got a listing of 140 homes in his $75,000 to $95,000 price range, narrowed the choice to four after driving by all of them, and eventually bought one through Williams.

Another Williams customer, William Mandel, is the kind of home buyer brokers fear might become typical if multiple listings are open to the public. The young restauranteur is buying pages of the listing because he plans to approach home sellers directly, to pursuade one to drop his broker and make his own discount deal.