The Florida man who shot and killed black teenager Trayvon Martin in an incident that triggered nationwide civil rights protests has placed his gun on a second auction website after it was dropped by a firearm auction site that said it wanted no part in the sale.

Key points: Zimmerman presses on with gun auction

Zimmerman presses on with gun auction Bidding on gun is to stay at $6,800

Bidding on gun is to stay at $6,800 Proceeds were set to fight the Black Lives Matter movement

The one-day auction of George Zimmerman's pistol had been scheduled to start at 11:00am (local time), with a minimum price of $US5,000 ($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 Mr Zimmerman.

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

Mr 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 $US5,000 starting bid, according to the newspaper.

George Zimmerman was found not guilty of killing teenager Trayvon Martin. ( Reuters: Joe Burbank )

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

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

Mr 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 Trayvon Martin Foundation "has no comment on the actions of that person that murdered Trayvon", Martin's father, Tracy Martin, said in a statement.

Mr Zimmerman, who was a neighbourhood watch volunteer at the time of the incident, said the shooting was in self-defence.

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

Proceeds to help counter Black Lives Matter movement

US President Barack Obama had said after the acquittal of Mr Zimmerman 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.

Mr Zimmerman has said that he plans to use a portion of the proceeds to fight Black Lives Matter, a black rights movement that grew out of the incident, and to counter "violence against law enforcement officers".

Proceeds were also be used to help fight Democratic presidential hopeful Hillary Clinton's "anti-firearm rhetoric", he said.

"I am honoured [sic] and humbled to announce the sale of an American firearm icon," he said in the description.

"The firearm for sale is the firearm that was used to defend my life and end the brutal attack from Trayvon Martin."

The number from the Martin case is written on the pistol in silver permanent marker and "many have expressed interest in owning and displaying the firearm", he added in the description.

Mr Zimmerman, who has had brushes with law enforcement since his acquittal, was the target of an attempted murder by Matthew Apperson in a Florida road dispute in May 2015, according to prosecutors.

Reuters