WASHINGTON (REUTERS) - The Florida man who shot and killed black teenager Trayvon Martin, triggering nationwide civil rights protests, put his gun on a second auction website on Thursday after it was dropped by a firearm auction site that said it wanted no part in the sale.

The one-day auction of George Zimmerman's pistol initially had been scheduled to start at 11am (11pm Singapore time), with a minimum price of US$5,000 (S$6.800).

But the website, GunBroker.com, said in a statement that it had rejected the user-generated listing created late on Wednesday and had no relationship with Zimmerman.

"We want no part in the listing on our website or in any of the publicity it is receiving," the statement said.

Zimmerman told the Orlando Sentinel the auction site was not prepared for the web traffic and publicity surrounding the sale. He said the gun was for sale at unitedgungroup.com with the same US$5,000 starting bid, according to the newspaper.

In the earlier auction listing, Zimmerman had described the Kel Tec 9mm pistol as "a piece of American history."

He told a local television station it was his to do with as he pleased, despite receiving death threats over his plan to sell it.

"What I've decided to do is not cower," he told Orlando broadcaster WOFL. "I'm a free American. And I can do what I like with my possessions."

Zimmerman said the US Department of Justice recently returned to him the gun he had used to kill the unarmed Martin on Feb. 26, 2012.

Zimmerman was acquitted of second-degree murder and manslaughter charges in the incident, which sparked civil rights rallies and brought scrutiny of Florida's controversial "stand your ground" law.

The law allows a potential crime victim who is "in fear of great bodily injury" to use deadly force in his home or in public places. It also says there is no obligation to retreat.

Zimmerman, who was a neighborhood watch volunteer at the time, has maintained that the shooting was in self-defense.

Martin's family said the teenager was simply passing through the residential area on his way home from a convenience store.

President Barack Obama said after Zimmerman's acquittal that Martin "could have been me, 35 years ago" and urged Americans to understand the pain African Americans felt over the case.

Daryl Parks, a lawyer for Martin's family, said the offer to sell the weapon was offensive, but that the family remained focused on their work advocating against gun violence.

"It shouldn't be a distraction to what we're doing," Parks, who is also chairman of the Trayvon Martin Foundation, said in a telephone interview.

'SOCIOPATH'

"George Zimmerman" quickly became the top trending term on Twitter in the United States on Thursday, with many users on the social media site expressing shock and revulsion.

"The only people worse than George Zimmerman are the people who bid on that gun," tweeted journalist and columnist Lyz Lenz.

National Review columnist Charles C. W. Cooke said Zimmerman"may have acted legally, but the man is a sociopath."

On the GunBroker.com auction site, Zimmerman said he planned to use part of the proceeds to fight Black Lives Matter, a movement that grew out of the incident, as well as to counter "violence against law enforcement officers."

Proceeds would also go towards fighting Democratic presidential hopeful Hillary Clinton's "anti-firearm rhetoric," he said. "I am honoured and humbled to announce the sale of an American firearm icon. The firearm for sale is the firearm that was used to defend my life and end the brutal attack from Trayvon Martin," Zimmerman wrote on the site.

The number from the Martin case is written on the pistol in silver permanent marker. The auction listing included a photo of the gun being held up in court by a law enforcement officer during Zimmerman's murder trial.

Zimmerman claimed in the description that many parties had expressed interest "in owning and displaying the firearm, including The Smithsonian Museum in Washington D.C."

The Smithsonian refuted his claim in a post on Twitter, saying it had never expressed interest in the gun, and had "no plans to ever collect or display it in any museums."

In a phone interview with Orlando's WOFL on Wednesday, Zimmerman dismissed criticism of the auction.

"They're not going to be bidding on it, so I couldn't care less about them," he said.