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Houses that are never even listed are selling fast, experts say. They’re called “whisper listings.”

Whisper listings are residential properties released directly to a broker who only allows specific clients to view the home. It’s not advertised publicly or listed on the area’s multiple listing service (MLS).

It’s an attractive option for sellers who don’t want to go public, don’t want their privacy disturbed or are hoping for a quick sale. But it leaves out other buyers who aren’t in that small circle of clientele and makes the housing supply even more limited for those searching the public market.

The controversial practice has been around for several years, but it’s been growing in popularity during this hot housing market, especially in places like Florida. According to Redfin, the number of whisper listings has increased from 2.4% to 4.0% since November 2019, and there are no signs of it slowing down.

“Nothing like this ever in 18 years that I’ve been doing this job. It is absolutely incredible what is going on here,” Palm Beach, Florida, real estate agent Liz Hogan said.

The National Association of Realtors has discouraged the practice, according to the Wall Street Journal, adopting a new policy that requires its agents to list a property publicly within one business day. But there are exceptions, of course.

Meanwhile, existing home sale prices in the U.S. hit a new record last month with the biggest jump year over year in the National Association of Realtors’ history. A shortage of properties and low interest rates, among other things, have fueled the housing boom.