short phrases are not protected by copyright. But ideally, you want a consent before republishing in a book.
Gobbledygook dept. Copyright law requires that you have a written agreement granting you the right to publish other people's work and electronic agreements such as clickwrap agreements are fine (as long as the user consents before posting). At Nolo, where we work, where we make an effort to avoid gobbledygook, a statement such as "By posting this comment you are granting [website owner] the non-exclusive right to reproduce it online, and in print and digital publications," doesn't seem like gobbledegoo (as Sylvia Plath referred to it) to us. This notice should be posted prominently in the space where the user is posting so that publishing the comment acts as a form of "click-to-accept" agreement. It's a good idea to save a screenshot of your site with the date you instituted the permission policy (and save a HTML version of the site as well).
Using screen names. You should be okay posting screen names as they don't identify the poster, but issues may arise when reproducing people's real names. If you want permission, you'll need to tweak your goobledegook (note the accepted alternate spelling) to state "... and you grant permission to reproduce the name (whether screen name or real name) used in connection with the posting of your comment."