KEY WEST, Fla. — The firearm that George Zimmerman used to shoot Trayvon Martin in 2012 went on sale for $5,000 on Thursday, generating a torrent of denunciations and extending the notoriety of a case that galvanized the nation.

By Thursday night, however, efforts to sell the gun had fizzled.

For Mr. Zimmerman, the former neighborhood watch volunteer who was acquitted of murder charges in the killing of Mr. Martin, a black 17-year-old, the announcement of the sale was the latest in a string of episodes that attracted national scrutiny. Mr. Zimmerman set off outrage in September by retweeting an image of Mr. Martin's corpse.

Mr. Zimmerman, 32, announced that he intended to auction the handgun used in the killing in Sanford, Fla. He said that the firearm had recently been returned to him by the Justice Department and that he hoped to use some of the proceeds to fight violence against law enforcement officers by members of Black Lives Matter; to ensure “the demise” of the career of Angela Corey, the prosecutor who put him on trial; and to counter Hillary Clinton’s “anti-firearm rhetoric.”

Mr. Martin’s mother, Sybrina Fulton, has been on the campaign trail with Mrs. Clinton, the likely Democratic nominee for president.