if i buy a banksy greeting card then i most definitely assume that it is licensed because i would not consider that they'd be allowed to do so without licensing.
if that is the issue though, wouldn't it be enough for banksy to put a price on the license?
you want to sell banksy greeting cards, you need to get a license from banksy. he just needs to put the price high enough to make it worth his while, and/or to make it unprofitable for the would-be licensee.
Copyright law is automatic, but it does complicate things if the original author of a copyrighted work is unknown or difficult to contact. For licensing, you need a name and address of an entity to license from. The whole idea behind Banksy's art is that he's anonymous, so precludes the idea of licensing his work.
> you want to sell banksy greeting cards, you need to get a license from banksy
That is exactly the sort of thing opening a store with tangible goods will allow.
banksy is using his mark, and so is disney. i am making the difference between disney movies and disney merchandise. how would disney merchandise not infringe on disney movies if they are created by independent companies without permission?