Customising branded products

Hi, I was wondering if anyone could tell me where I stand on customising branded products?

Things such as products with marvel or Disney characters on, is it okay to add someone's name and sell it for a profit?
I have seen lap trays in wholesalers which I would like to customise.
If so, how would you recommend advertising these to not infringe any laws? "Marvel lap tray with customization"?