If the characters are trademarked then it's not legal unless you have written permission from the trademark owner. If the pages are from a coloring book then that would violate the authors copyright.
If it didn't come from you, your design, your creative process, then you can't reproduce it and sell it has microstock. If caught you could face lawsuits from the trademark/copyright holders as well as be responsible for any legal fees that the site has to pay.