George Bramlett, the owner of Bargain Pawn in Las Vegas, said Christopher Dorner sold the gun to him a month before he went on a killing rampage and committed suicide after a massive manhunt, the Los Angeles Times reported Friday.
Bramlett gave Dorner $50 for the .38-caliber revolver and the shop owner said bidding had reached $585 by Friday afternoon, with 18 bids made.
Money from the auction will be donated to the victims' families, Bramlett said.
The online listing is accompanied by a surveillance camera image of Dorner and a copy of the sales receipt with Dorner's signature.
The head of a police group raising money for a fallen officer calls the auction "tasteless."
Sgt. Brian Smith, president of the Riverside (Calif.) Police Officers Association, said, "We're not interested in the money."
He suggested the gun be destroyed.
Bramlett thinks he can raise $25,000 in the two-week auction.
"It's a piece of history," he says, "even if it's a bad history."