You can sell public domain materials all you want. And whenever anyone else copies stuff in the public domain, even if they copy it from your copy, they're not infringing copyright.
But be careful as when someone publishes something public domain they often have things that are not public domain in their version. They can "fix" errors, add artwork, introductions and so on.
Yes, of course, the added content would not be in the public domain. Although it should be note that certain changes such as fixing grammatical errors/proofreading would not be considered copyrightable.