Unless the material is copyrighted, and acknowledged as such, it is
probably like any other medium, and in the public domain. So the
concerned should beware before they post; the internet is an open
domain, and some people will take without rendering credit.
If there are legal issues, I'm not aware of them - at least for the US.
Readers are encouraged to comment.
Robin Wilson's column in the Math. Intelligencer has earned him the
respect of many, myself included. In fact, my interest in mathematical
philately began with his material.
Al Barron
Metuchen, New Jersey