"An important thing mentioned is that he never licensed or agreed to merchandise the characters. That means that anything related to Calvin and Hobbes that isn't one of the books is a knock off and Watterson gets nothing for it."
* * *
"Almost anything - there were a couple of calendars and 'Teaching with Calvin and Hobbes' which are legit.
But on the whole you're very right, and especially any stickers involving Calvin pissing on something. :)"