This site asserts the trademark to Banksy’s name and images is held by the artist, and is not transferable to any third party.
The artist would like to make it clear that he continues to encourage the copying, borrowing and uncredited use of his imagery for amusement, activism and education purposes. Feel free to make merch for your own personal entertainment and non-profit activism for good causes.
However, selling reproductions, creating your own line of merchandise and fraudulently misrepresenting knock off Banksy products as ‘official’ is illegal, obviously a bit wrong and may result in legal action. In the event of prosecution all funds will be donated to charity.