I'm a little confused here.
If I am reading this right, the print was sold by Banksy to a private collector in 2015. But Banksy has some objection to it being auctioned, even though he sold it. Was a no-auction clause part of the sale terms? Was the owner notified of this term?
Selling this one last does seem to have been designed as a show stopper. If it was Banksy's 'comment' that art auction values are excessive and obscene...well, they have been reinforced by the stunt, so is the real message that Banksy has formally sold out?