Are you serious? I'm fairly sure you're protected by the first amendment for selling MLK-Hitler posters. In fact, you can Google for those items now. Here's one:
http://www.redbubble.com/people/artofmewa/works/8484381-the-...
http://www.redbubble.com/people/artofmewa/works/8484381-the-...
As for Trademark... what is the trademark here, and how would anyone be confused by an MLK-Hitler poster? I don't think anyone is going to buy an MLK-Hitler poster thinking that they are getting MLK-brand doughnuts (or even MLK-Hitler brand doughnuts).