Newcastle sent out an email to potential buyers and posted a , which said: "The Board of Newcastle United can today confirm that the Club is for sale at the price of £100m.
"Interested parties should contact Newcastle United at admin@nufc.co.uk [or Keith Harris at Seymour Pierce] for further details."
The club declined to make any further comments.
A fake Newcastle United for sale item was posted on eBay, but this has now been taken down. Similar groups have sprung up on Facebook.
John Beresford, former Newcastle defender and current ITV football pundit, said the email made the team look like a "laughing stock". The Sun equated the email as if Ashley had put the club up for sale on eBay.
Beresford told The Sun: "The season was like a slow car crash with no brake being applied. But now people are just laughing.
"Can anyone imagine Manchester United asking people to email them bids for the club?"
Newcastle was relegated to Coca-Cola Championship league following their bottom three finish in the 2008-09 Premier League season.
According to The Sun, rival fans at Sunderland, who finished 16th this year, inundated Ashley's email inbox, kicking the squad while they were down with cheeky offers.
One fan, Sam, wrote: "Hi, I am prepared to offer an old copy of Shoot magazine from 1981 and the last six stamps I've got from a book of 10. I'll go to ten if you can offload Mickey Owen."
Another said: "Sir... Does the price include the fat bloke as well? I can't afford both."
Another quipped: "I would like to offer £10 for NUFC. This is my full and final offer.
"I am currently in a management role in the Peterlee Sunday League. If I'm successful, I'll bring my entire playing squad with me.
"They are a damn sight better than the crap on display at the moment. Yours in Sport."