Although she has given the buyer a tentative nod, Jill Greenberg, one of his neighbors, is not as eager to sell. Ms. Greenberg was the original holdout shareholder during the first round of talks with BH Chelsea. Her lawyer, Mr. Bailey, who also represents Mr. Selbert, said the only way she will agree to sell is if the investor meets her price. “We have a number,” he said. “They know the number.”

Because Ms. Greenberg owns the penthouse, she has laid claim to the roof, and has consequently challenged the calculations that determined the percentage of shares accumulated by the buyer. So there has been some question as to whether the investor will have enough shares to collapse the co-op without her participation.

Tom Brady has seen this sort of thing quite a bit — but from the other side of the negotiation. A salesman at Town Residential, he has been working with several developers trying to convince shareholders in various co-ops to sell. “The developers give me the address, tell me what they want to offer and a ceiling, and then send me out with my marching orders,” he said.

Mr. Brady employs a variety of strategies, including what he calls “guerrilla marketing,” which involves, among other things, getting inside buildings to slip handwritten notes under people’s doors.

“I write to show them it isn’t just a form letter,” he said, adding: “I tell them I am not looking for an exclusive, but that I am representing a developer who wants to buy their home and is willing to pay top dollar. I tell them we can pay all cash, close in three months and help them find a new apartment.”

He is currently working on three such deals, he said, but all have yet to close, and until then he is reluctant to discuss specifics. “Every deal is totally off market,” he said. “And the developers don’t want anyone to know what they are working on.”