The first edition of Harry Potter And The Chamber of Secrets went under the hammer at a Hansons Auctioneers sale held at Bishton Hall, Staffordshire, today.
The book was snapped up by a private international buyer, the auction house said.
Mark Cavoto, 51, a businessman who owns around 1,500 books from the series, ordered the collector's item off Amazon for 1p, plus postage, without realising it was signed.
The father-of-five has bought and sold around 3,000 Harry Potter books over the years.
He started trading when his daughter, Holly, now 25, began collecting them and he realised they could be sold at a high asking price.
Mr Cavoto, from Buxton, Derbyshire, said: "When Holly was a child, she loved Harry Potter books and started her own collection.
"About 10 years ago, we decided to sell some of her older books and three of them were snapped up within 20 minutes on eBay for £9.99 each.
"I'm a businessman with a shop in Buxton and I knew this was a business opportunity.
"I checked the ISBN numbers and sourced the same three books second-hand on Amazon, bought them for a penny each plus postage and sold them in minutes for £9.99 each on eBay."
Mr Cavoto, who helps collectors source items through his business Partworks Collectables, said he was buying around 10 books a day when he came across the signed copy with Holly.
He said: "Holly liked to open all the Jiffy bags.
"One day she glanced at a book she'd just opened and said, 'This is no good, it's got ink on it'.
"She carried on opening the others then went back to it.
"She stared at me and said, 'Oh my God, it's been signed by JK Rowling - and it's a first impression'.
"I thought she was messing with me but it was true.
"It was signed 'To Alexandra, JK Rowling' and was a 1998 hardback first edition of Harry Potter And The Chamber of Secrets, the second novel in the series."
Mr Cavoto has struck lucky with the books before, when he spent one pence on a copy of Harry Potter And The Philosopher's Stone from the second print run, which sold for £4,600.
He said: "It's as rare as the 500 books in the first print run.
"I decided to advertise it for £5,000 and an Australian dealer offered £4,600 for the book I'd paid a penny for.
"I snapped his hand off and used the money to start Partworks Collectables."
Mr Cavoto said collectors like to have the same book from every print run to complete their haul.
In October, Hansons, based in Etwall, Derbyshire, hosted the bidding on a rare first edition of Harry Potter And The Philosopher's Stone - the first book in the series - which eventually went for £57,040.
The book was in high demand because only 500 were printed - most of which went to schools and libraries.
Reacting to the latest sale, Jim Spencer, Hansons book expert, said: "That's some return for spending one penny.
"What a great result for an incredibly lucky online purchase.
"I'm delighted for both the seller and the new owner, who is a private international buyer.
"The Harry Potter phenomenon forges on."
He added: "We've had phenomenal success with Harry Potter books this year - so much so I'm still getting about 50 emails a day from people who think their copy may be valuable."