Even though they deny it, I suspect the auction house (Sotheby’s) was in on the stunt. It was the last auction of the day and the painting was hung on the wall instead of held by an easel, which according to news reports was unusual. And the thick frame and weight of the thing, I think, would have tipped off Sotheby’s even if they weren’t in on it. And what about the seller — according to news reports, it wasn’t even Banksy, but was, instead, someone he sold the painting to several years ago (which seems unlikely to me).
As always with Banksy, there’s lots of mystery and controversy surrounding it, which is, of course, intentionally manipulative and at the heart of his work. I suspect the partially shredded painting is worth more now than before since this whole thing has made it famous. Just for the record, I’ve never liked his work.