You aren't allowed to use copyright images as far as I know. I imagine sellers using copyright images are pulling them off of Google image search. I'd be very careful about making/selling copyright material because it's against the law and I'm also fairly positive it's against Etsy rules. What I have seen people do (and have done myself) is to make images 'inspired' by something. For instance, I have a Spiderman 'inspired' paper pack that I made and sell. It has no copyright content in it but has all the Spiderman colors, webs, patterns, etc that are reminiscent of Spiderman. No one can call me on copyright because there are no words, phrases, or images in the product that are copyright.
