“I’m a free American,” he told Fox 35 in Orlando. “I can do what I like with my possessions.”
Zimmerman was acquitted of murder in 2013 a decision which lead to the protests that help spark the “Black Lives Matter“ movement.
The auction began on Thursday morning on GunBroker.com with a starting bid of $5,000.
There are currently no bids.
In the description, he claimed the gun was used to “defend my life" and was "a piece of American history".
Zimmerman said he had received offers for the gun, but “the offers were to use the gun in a fashion I did not feel comfortable with.”
The move has provoked widespread disgust.
Zimmerman's description of the gun ends with “Si Vis Pacem Para Bellum,” Latin for “if you wish peace, prepare for war.”
Since his acquittal Zimmerman has caused outrage by selling prints of his painting of a Confederate flag, calling President Barack Obama an “ignorant baboon” and tweeting a photo of Martin’s body.
In 2014 he attended a gun show at which he signed autographs.
According to the Daily News only a handful of people turned up to get an autograph during the six-hour session. Speaking to the newspaper, Zimmerman said he was there to meet his “supporters”.
One woman who attended the show told the Daily News: “It was pretty weird to meet him in person — and wow, he has gained a lot of weight!”