Nationwide protests erupted over the 17-year-old's shooting in 2012, in what the watchman George Zimmerman said was self-defense but which the teen's family and friends claimed was a murder driven by racism.
Zimmerman's trial and subsequent acquittal exposed deep racial fissures in US society.
Bids for the Kel-Tec PF-9, a 9mm pistol, began at USD 5,000 on Thursday -- already far above the market value of a secondhand weapon -- and had reached USD 65,039,000 by today to go in the sale.
US reports said a bidder using the screen name "Racist McShootface" made several offers of around USD 65 million overnight, though the account was later deleted.
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The top bid early today belonged to a user identified as Craig Bryant.
It was difficult to verify the validity of the bids, with participants using pseudonyms and seemingly bidding against themselves. The website does not request downpayments for bids.
"Newsflash people, we do not post auctions. Our members are solely responsible for that," tweeted United Gun Group, which calls itself "the fastest growing social market place dedicated to the firearms community."
Another's chosen screen name was "Joe Doe," close to the "John Doe" placeholder name used by people who want to keep their identity anonymous, especially in legal action.
Website users sparred in the comments section, with some denouncing bids that were clearly fake.
"This just goes to show people how stupid yall really are," said a user who went by the pseudonym "Quit trolling it's not worth it.