(CNN) George Zimmerman tried to sell the gun purportedly used to kill Trayvon Martin on two different auction websites Thursday.

Both times, the websites kicked him off.

Zimmerman said he used the gun to kill Martin, an unarmed African-American teen , in Florida four years ago. He claimed self-defense and was found not guilty.

Bidding on the gun was supposed to start Thursday morning on GunBroker, but the website sent out a message in the afternoon saying it had rejected the idea.

"Our site rules state that we reserve the right to reject listings at our sole discretion, and have done so with the Zimmerman listing," the GunBroker statement said. "We want no part in the listing on our web site or in any of the publicity it is receiving."

Zimmerman told CNN he moved the auction to another website, giving a different reason than GunBroker did.

"Unfortunately, (the site) was not prepared for the traffic and publicity surrounding the auction of my firearm," Zimmerman said in a text. "It has now been placed with another auction house."

Zimmerman then tried to sell the gun through United Gun Group, an online marketplace for firearms. It listed the weapon at a starting bid of $5,000, the same as on the previous site.

But United Gun Group put out a statement Thursday night, saying the gun will not be sold on its site.

"As an organization, we stand by the rule of law and, while no laws have been broken, we do not feel like it is in the best interest of the organization to continue to host this sale on our platform," it said.

"Our mission is to esteem the 2nd amendment and provide a safe and secure platform for firearms enthusiasts and law-abiding citizens; our association with Mr. Zimmerman does not help us achieve that objective."

'Piece of American history'

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On both websites, Zimmerman provided some description of the gun and his motivation.

"I am honored and humbled to announce the sale of an American firearm icon," the websites said. "The firearm for sale is the firearm that was used to defend my life and end the brutal attack from Trayvon Martin on 2/26/2012."

The post bragged about the gun's popularity, describing it as a "piece of American history." The posts were later inaccessible when CNN tried to view them.

Before rejecting Zimmerman, United Gun Group owner Todd Underwood described his website as a "social market for firearms community." Services are free and no money is made, he said.

The posts apparently written by Zimmerman said museums such as the Smithsonian Institution have "expressed interest in owning and displaying the firearm."

But the Washington-based Smithsonian denied it had any interest in the weapon.

"We have never expressed interest in collecting George Zimmerman's firearm, and have no plans to ever collect or display it in any museums," it said in a statement.

'I'm a free American'

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Zimmerman's plans to auction the weapon surfaced Wednesday.

In an interview with WOFL-TV in Orlando, Zimmerman said he decided to sell the weapon and move past it.

"I recently received it back from the Department of Justice," he said. "They took it after my trial, after I was exonerated."

Zimmerman told the station he has had death threats since he put the gun up for sale.

"What I've decided to do is not cower," he said. "I'm a free American. I can do what I want with my possessions."

Proceeds from the gun's sale were supposed to be used to "fight [Black Lives Matter] violence against law enforcement officers," according to the original post that later disappeared.

Martin's family: Focus is on ending gun violence

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Martin's family declined to comment on the purported sale of the gun, saying the Trayvon Martin Foundation is focused on ending gun violence in the United States.

"This election season, we are laser focused on furthering that mission," Martin's family said in a statement.

"As such, the foundation has no comment on the actions of that person."

The former neighborhood watch volunteer admitted to shooting Martin in February 2012 but said he was defending himself after the teenager attacked him

His acquittal sparked nationwide debate and protests in a case that was racially charged from the start.

Critics of the verdict have described Zimmerman as an overzealous wannabe police officer who racially profiled Martin and shot him down.