The buyer is a female collector from Europe, the auction house said.
News that the painting self-destructed went viral even before an Instagram account tagged to Banksy suggested he was behind the incident.
"Banksy might think that by destroying his art he's undermining capitalists who buy it, but he's wrong", commented Mikael Faujour of the specialist French art magazine Artension. Would its new, half-shredded form be worth less money or more, given its elevated status after the global media attention?
"It's a performance in the line of Marcel Duchamp's Ready Made", he said speaking of a term coined by the French artist in 1915 to describe a sometimes modified, but always common object, not usually thought of as a work of art.
"Rauschenberg erased de Kooning's drawing; Gustav Metzger and Jean Tinguely pioneered auto-destructive art; Michael Landy shredded all his possessions; and what about Banksy himself and his corrupted oils?"
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The buyer has agreed to take possession of the piece at the $1.4 million price despite the shredding, Sotheby's said.
At Sotheby's, to whom the work belonged to since 2006, claim they didn't know about hidden in the painting device.
But the freakish turn seemed to have only bolstered the piece's legacy - and value - with Alex Branczik, Sotheby's head of contemporary art for Europe, dubbing it "the first artwork in history to have been created live during an auction". It has also been renamed "Love Is in the Bin" (2018)-an indication that the artist sees it as a new work.
The new work has been given a new title, Love Is In The Bin, and has been granted a certificate by Pest Control, Banksy's authentication body.