I honestly don't know why anyone at the auction, who was interested in that piece, thought there would be no trick or strings attached, or why Sotheby's didn't suspect anything after investigating Banksy. The wiki excerpt shown by Google says "Banksy is an anonymous England-based street artist, vandal, political activist, and film director." It then goes on to call his works satirical and focused on dark humour.

Anyone willing to spend £1,000,000 on a piece of art by Banksy should know enough about him to imagine that he'd hate auctioning his work to the rich at an elite establishment like Sotheby's.