I wouldn’t allow any site merchandise if I were you. Profiting off of other company’s brands is very dicy legally and is a lawsuit waiting to happen.
We wouldn’t be selling them. Maybe if everybody chips in a buck for a limited run that covers costs? I’ve seen oodles of shirts at conventions over the years that are definitely not licensed. Even more on Ebay.
Note to self. Must buy a I survived a Mok Concert t shirt.