I know it is illegal to make a Disney cake and sell it, but what if I made a vanilla cupcake with vanilla buttercream icing, and put two mini oreos on it for Mickey Mouse ears. Is that illegal? Also, what if I put Frozen toothpicks in my cupcakes?
The mini oreos would be tricky, but as long as you're not plastering "Mickey Cupcakes" all over your website and advertising them that way it would probably be okay. The Frozen picks are trickier since they could come under "first sale doctrine." If you were making the toothpicks yourself and drawing the characters, or if you were photocopying them then that wouldn't fall under that, but I think that if you buy them you're allowed to sell them as-is. https://en.wikipedia.org/wiki/First-sale_doctrine
I'm not an attorney, and my opinions are speculation based on what I think I understand about first-sale, though! It's always safer to have the customer buy trademarked and copyrighted items and handle them themselves without you getting involved in it.