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Alison Young, a professor in criminology at the University of Melbourne who has studied street art and Banksy for years, said this was far from a prank gone wrong.
She said Banksy would have known all along what effect the stunt would have on the value of his work, and it was part of his ongoing exploration of the commercialisation of art.
"It's a stencilled piece that can be easily replicated. I think Banksy would find this all very amusing," she said.
Professor Young also suspects that Sotheby's and potentially both the buyer and seller were in on the joke.