Play your cards right to pick up a Hockney for just 99p

The catch, barring the deciphering of the occasional clue or the ability to spot genius at work, is that nobody will know the identity of the artist until they buy the postcard and turn it over to see the signature on the back.

More than 600 pictures have been included in The Big Art Secret, a fund-raising sale for St Wilfrid’s Hospice in East Sussex, which also features drawings by hundreds of unknowns, including patients, a girl aged four and a 100-year-old retired teacher.

More than a dozen celebrities have supplied their own creations, including world-famous artists David Hockney and Tracey Emin, Prime Minister David Cameron, former tennis star Greg Rusedski, comedian Ronnie Corbett and writer Frederick Forsyth.

Ms Emin has donated four postcards, said to be in her trademark style, while Mr Hockney’s work, which may not be so recognisable, is described as “an ink and colour crayon drawing on a postcard”.

The cards are being shown at a local gallery and sold through the internet auction site eBay, where the sale will close on Sunday. Bidders will not only be supporting a good cause - proceeds will go towards the charity’s £5.3m campaign to build a new hospice – but may pick up a Hockney relatively cheaply and, depending on the price, make a sizeable return on their investment.

Yorkshire-born Mr Hockney is widely regarded as Britain’s greatest living artist and his work has been sold for vast sums. In 2009, a 12ft-by-6ft work from his early Californian period entitled Beverly Hills Housewife fetched £5.2 m at Christie’s in New York.

The chief executive of St Wilfrid’s Hospice, Kara Bishop, said: “This is an opportunity for anyone, not just art collectors, to support our charity and stand to gain substantially from it, be it in terms of owning a valuable artwork or from the enjoyment we can all get from a piece of art that truly strikes us as beautiful or moving.

“The auction will appeal to art collectors and complete newcomers to the art world but the beauty is, with artists spanning 96 years and from household names to hospice patients, nobody will really know which ones are by a famous artist until after the virtual gavel falls.”

The sale also throws up the intriguing possibility of judging the artistic talents of people who have made their name in other fields. From the world of politics, Mr Cameron’s contribution show alongside pieces from Deputy Prime Minister and Sheffield Hallam MP Nick Clegg and Liberal Democrat peer Lord Ashdown. However, the Cameron and Clegg postcards are not originals but limited edition prints.

Asked who was the better artist between the country’s two most senior politicians, the head of community engagement at the hospice, Charlotte Nuckhir, said: “I would say Clegg’s is probably better than Cameron’s; there’s slightly more detail in it. They are quite fun but they are not the most remarkable pieces in the auction, I must say.”

She added: “My favourites are Ronnie Corbett’s. He’s done two and obviously put a lot of thought into them. He’s also written a letter that accompanies each postcard, so whoever succeeds in bidding for them will get a handwritten letter explaining the picture, which I think is a really nice touch. He’s also put a clue in one of them.”

Mrs Nuckhir secured what is likely to be a rare drawing by Mr Rusedski, the former British tennis number one, when she was running a stand for the hospice during a veterans’ tournament at Eastbourne in June.

“He came to the stand and I asked him if he was creative,” she said. “He said he wasn’t so I said ‘Can you draw anything at all’? and he said ‘I can probably do something’. His is very obvious.”

Mrs Nuckhir said she was surprised but delighted at how the auction had taken off since the idea was first put forward by Charles Grimaldi, a volunteer ambassador at the hospice, six months ago.

“It’s absolutely amazing, beyond everything we ever imagined,” she said. “It’s the biggest charity auction eBay have ever done.”

Matthew Rowe, artistic director at the Towner art museum, where the exhibition is being held, added: “The response not only from the art world but from all walks of life has been phenomenal. But the twist is in the secret – by obscuring the market value, who knows the true financial worth of each postcard?”