When street artist Banksy first started selling screen prints online, they became a popular target for forgers looking to make a few bucks duping unsuspecting fans. It quickly got out of hand and must have been infuriating for both Banksy and his followers.
Normally this is where galleries begin to flex their authority and declare which prints are authentic and which are fake. Galleries sometimes print certificates to reassure customers that the large lumps of cash they are parting with is for the real thing, not one of those dirty fakes doing the rounds. I don’t have large lumps of cash, so I’ve never seen one, but I imagine they say something like this:
The enclosed screen print is totally by Banksy. By authority of some gallery corporation.