Online auction firm eBay has said it has a "genuine first print, first issue hardback" edition copy of Harry Potter and the Prisoner of Azkaban.
The Prisoner of Azkaban film has proved a global box office hit
The sellers detail errors on the title page and first page and said it was unread and sealed in an airtight box since buying it in 2000 for £600.
The sale price has reached £1,200 so far, with three days of bidding to go.
Publishers Bloomsbury told BBC News Online it was impossible to verify the copy's authenticity without seeing it.
The sellers have decided to auction it for money to invest in their business after watching its value soar over the past four years.
The sellers' entry reads: "This was a really good investment for us."
They say the book can be proved to be a first state copy because the author's name on the reverse of the title page is printed as Joanne Rowling.
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Also, on the first page of the story there is a dropped line of text between the word "burnt" and "so much".
The sellers added the presses were stopped part-way through the initial run of 10,000 copies to correct these mistakes, making the book even rarer.
A spokeswoman for Bloomsbury said they would have to examine the book close-up before they could authenticate the sellers' claims.
In January this year, an uncorrected proof copy of the first Harry Potter book - with the author's name spelled wrongly - sold for £1,250 at auction.
A signed first edition of Harry Potter and the Goblet of Fire was sold by the author's father for £27,500 at auction in 2003.