According to news. com. au, Bradman wore the cap as captain of the 1948 Invincibles Ashes tour of England and it was to have been sold with a letter written by Bradman himself confirming its authenticity.
Shortly before the auction, auctioneer Charles Leski of Charles Leski Auctions, said the cap was one of the most valuable items of sporting memorabilia ever offered for sale.
The cap was passed in without any bids at the reserve price of 450,000 dollars.
On Monday, Leski told the Nine Network he expected the cap to fetch more than 600,000 dollars.
Following the auction, Leski said there had been a private offer to buy the cap, but because the offer was less than reserve he would need to negotiate with the vendor.