Condo sales in Long Island City are suddenly soaring, thanks to Amazon.com Inc.’s decision to open a headquarters in the Queens, N.Y., neighborhood. One local brokerage firm reported it sold nearly 150 units over four days last week, about 15 times its usual volume.

Joining the rush were at least three Amazon employees who recently bought homes in Long Island City, according to real-estate brokers. Two of them signed contracts at the Galerie, a new 11-story condo building with a pool and interior courtyard, said Brendan Aguayo, a senior managing director at Halstead Property Development Marking, which is selling units there.

The two employees decided to the buy units just before the first press reports surfaced that Amazon was likely to choose Long Island City for its new headquarters, Mr. Aguayo said. The buyers now live in New Jersey and Queens, he added.

After publication, an Amazon spokesman said Wednesday the company had no evidence that any employee who may have made a property purchase before the announcement had any advance knowledge of the location selections.

While employees aren’t permitted to buy and sell stocks based on nonpublic information, several real estate lawyers said they were aware of no such prohibition for real-estate transactions.

Amazon announced on Nov. 13 that it would open a new headquarters for 25,000 employees in Long Island City, which is sharing the second headquarters designation with Arlington County, Va. The first workers are due to arrive in New York early next year.

Brokers say they have already heard from Amazon employees working at the company’s Seattle headquarters who said they anticipated a transfer to the new office.

“My cellphone rang, and the area code was Seattle,” said Patrick W. Smith, a Long Island City based broker with broker Stribling. He said that as of Monday, five Seattle-based Amazon workers had reached out to him.

Prospective buyers listen to a sales pitch at the Galerie last week.

Several Amazon workers expressed interest in studios and one-bedroom units, generally priced below $1 million in Long Island City. Some were also looking at less expensive, low-rise neighborhoods including Astoria in Queens and Greenpoint in Brooklyn, Mr. Smith said.

On Friday, an Amazon employee based in Seattle made a deal to buy a one-bedroom condo at the Craftsmen Townhomes, a low-rise development in the neighborhood, said Eric Benaim, president and founder of brokerage Modern Spaces, which represents many condo developers. One-bedroom condos at the Craftsmen are listed for $800,000 to about $1.2 million.

There are no reliable figures on how many units have gone into contract in Long Island City in recent days, but many brokers described an unprecedented surge of buyer activity in the past weekend that surpassed a similar bump the week before.

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Mr. Benaim said that from Thursday evening through Sunday, his agents had recorded 147 deals in both new developments as well as sales of individual older condo units.

At a project known as the Corte, Mr. Benaim said that at least 15 units have gone into contract since the Amazon plan was disclosed—despite a price increase of about $20,000 per apartment that went into effect last week. He said prices will go up after another $20,000 price increase due to take effect this week.

At the Galerie, Mr. Aguayo said, 25 deals were done in the last two weeks, with additional offers expected for buyers who toured the showroom over the weekend.

The surge in sales comes amid a slump in the overall New York condo market during which buyers in Long Island City could count on making a deal below the asking price, brokers said. Because of the slowdown and a lack of urgency by buyers, many developers had been delaying opening sales until closer to the time that a building will be completed.

That reversed after news of Amazon’s impending move triggered an acceleration in the market. Developers are now holding the line on asking prices and planning increases.

A rendering of the Galerie, a new 11-story condo building where 25 deals have been done in the last two weeks, according to Halstead Property Development Marking’s Brendan Aguayo. Photo: Binyan Studios

Buyers who had been looking for months all wanted to buy at the same time, brokers said. If all the deals mentioned by brokers go through, they would account for nearly half of the current inventory on the market.

More developers are waiting in the wings. The 778-foot tall Skyline Tower, due to be the tallest building in Queens, is now planning to open its sales office in January rather than in the spring, when sales activity usually thaws, Mr. Benaim said. The tower is across the street from One Court Square, where Amazon will be taking office space until its new headquarters is built.

Several vans of Chinese-speaking buyers toured open houses for condos over the weekend, some brokers said, after some firms posted articles about the Amazon move on WeChat, a chat platform popular among Chinese speakers.

Apart from highlighting the rising home prices on condo developments near the East River, some posts included references to Long Island City-based Silvercup Studios, which produced the television series “Sex and the City,” a popular show in China.

And the postings didn’t play down the Amazon move. “A company of this heft will bring earth-shaking changes to the city it has chosen,” one WeChat advertisement said.

—Esther Fung contributed to this article.

Write to Josh Barbanel at josh.barbanel@wsj.com