So Banksy created a not-for-profit company, Pest Control, to sell and authenticate his works. The process is fiendishly clever, as Will Ellsworth-Jones writes in his book ‘Banksy: The Man Behind the Wall’:
Now, for £65 you can get your Banksy print authenticated. And just to keep the whole thing as jokey as possible, the authentication certificate has stapled to it half a ‘Di faced tenner’, a £10 note faked by Banksy with Lady Diana’s face on it. The tenner has a handwritten ID number on it which can be matched to the number on the other half of the of the note held by Pest Control.
The combination of the tear and the handwritten code makes for a delightfully difficult to reproduce method for verification.