You can sell works in the public domain. Go to your book store, I'm sure you can buy a copy of Frankenstein, which is in the public domain. The whole purpose of public domain is that no one owns the copyright, so there are no restrictions to anyone on what may be done with them. I could publish Frankenstein and sell it to people, but I can't stop other people from selling it.
If these documents really are under public domain, then JSTOR can't stop people from distributing the copies obtained from them. However, it is possible for JSTOR to claim copyright on the form of the distributed copies, but not the content (assuming the papers are in the public domain). That is, if they had a JSTOR logo on the papers, or something 'unique', JSTOR may have a basis for claiming copyright infringement. This is assuming they were actually obtained legally, and JSTOR didn't have a strange TOS.
If these documents really are under public domain, then JSTOR can't stop people from distributing the copies obtained from them. However, it is possible for JSTOR to claim copyright on the form of the distributed copies, but not the content (assuming the papers are in the public domain). That is, if they had a JSTOR logo on the papers, or something 'unique', JSTOR may have a basis for claiming copyright infringement. This is assuming they were actually obtained legally, and JSTOR didn't have a strange TOS.