And, most importantly, how to word this appropriately. MeatballWikiCopyright achieve 1 and 2. The third isn't enforceable. Third parties have fair use, anyway. More extensive copying should require the permission of the authors involved.
People feel kind of cheated if what they say is sold without their permission.
If I want to quote a document but don't know who contributed to it, I can only hope that they're still hanging around when I post a message asking if anyone has any objections to my copying it. I'm not very familiar with fair use guidelines. If possible, I'd like to facilitate that before the fact. However, in general it's safe to assume signatures are correct. Fair use is not simple, but it's pretty much common sense.