A stallholder who sold a piece of graffiti sprayed on his stall for £1,000 has told of his despair after learning it is a Banksy original that could fetch £500,000.

Sam Khan, 60, who has traded luggage and football scarves in Tottenham Court Road for 30 years, believes he was conned into handing over the mural for a fraction of its value.

When it appeared on his stall, the father of six from Highbury thought the image - showing a young boy holding a paintbrush - was the work of a vandal. But Mr Khan, who had never heard of "guerrilla artist" Banksy, noticed "trendy young people" were stopping to take photographs on their mobile phones.

He said: "I don't know anything about art. I've been on the stall all my life trying to make an honest living come rain or snow. I've had people coming up to me saying, 'How did you not know who Banksy is?' and, 'Why didn't you go on the internet?' I get up at 5am and I'm on the stall for 12 hours a day. I don't follow these things.

"The guy who bought it from me came with £1,000 cash and intimidated me into it. I was threatened when I asked for time to think and I had to deliver it to a storage depot in King's Cross.

"I've never been able to find that man again." Mr Khan paid £300 to have the steel panel removed with an electric saw and £300 for a replacement panel for his stall, leaving him with a net profit of £400.

He said: "I feel devastated. My stall is outside my bank and the manager came out with the Evening Standard to show me the story. He was joking that I'm always overdrawn and now I've lost £500,000.

"It's no joke. I have three young children from my second marriage and they need to be provided for. My health's not so good. I'd love it if Banksy came and did another graffiti work on my stall."

The mural, entitled What?, is being sold by an anonymous owner at Notting Hill's Bankrobber gallery. The reserve price is £230,000, but as Banksy works painted on public buildings rarely go on sale, experts believe it could make £500,000 at auction.

Robin Barton of the Bankrobber gallery said: "This is a one-off and important. It stood in the shadow of Centrepoint. Any cash above the reserve price will go to the Centrepoint charity [for the homeless]".

* A hundred sculptures by Angel Of The North creator Antony Gormley will be allowed to stay on Crosby Beach near Liverpool. The installation Another Place, cast from a mould of the artist's body, has been a hit with visitors, but Sefton council had said it could not stay due to safety fears. It has now changed its mind.