“They should give it to me and we’d sell it,” Mr. Keszler said.

In the course of Banksy’s New York run, Mr. Keszler said he was contacted by half a dozen building owners who inadvertently ended up with Banksy originals on their properties. (Stirring controversy, Mr. Keszler previously acquired Banksy pieces that had gone up in the West Bank city of Bethlehem.)

Mr. Keszler said he reached an agreement to buy one of the New York pieces — the image of a heart-shaped balloon dotted with bandages, which appeared in early October on a low-slung building in Red Hook, Brooklyn. Though the piece was tagged, Mr. Keszler said that he expected it to fetch between $200,000 and $400,000 when it goes on sale at an art show, Art Miami, next month. He said he already had the piece, which was carefully extracted from the wall by experts, though he would not say how much he paid.

Authentication of Banksy’s work is strictly controlled by an entity called Pest Control, which certifies bona fide pieces that the artist, who remains anonymous, intended to sell. Pest Control does not authenticate Banksy’s street work, saying he wishes it to remain public, a policy that also serves to sidestep admission of vandalism. But Mr. Keszler said that because Banksy posted his work to his website, people could ascertain whether a piece was real (that said, fake Banksys abound).

In any event, the New York Banksy balloons were deflated for space and are stored on the third floor of the Police Department’s building on Pearson Place in Long Island City, a police spokeswoman said. These balloons are not the department’s first: A six-story-tall Cat in the Hat was impounded in 1997 after it struck a lamppost and injured people at the Macy’s Thanksgiving Day Parade, but it was later returned to the department store.

In misdemeanor cases ending with a guilty plea, unclaimed property can be held for up to 18 months from the time it is brought in, Chief Trabitz said. After that, it is considered abandoned and is destroyed or auctioned off on the site PropertyRoom.com, which sells confiscated police goods. P. J. Bellomo, the company’s president and chief executive, said auction money went into the city’s general fund.

Yet Chief Trabitz did not anticipate a lucrative fate for the Banksy work, because it had been inventoried as something essentially valueless and was unlikely to be re-listed by the department as art.

“The item description is never changed,” he said. “You can’t redescribe it. It’s a balloon.”