I suggest that you contact the specific skinning author you're interested in these matters.
In general I would say typical Creative Commons apply: don't take stuff without a permission and this is even more important if you plan to make a profit out of them somehow. Of course some skins use work from the other skin authors so that makes it a bit more complicated (like for example some of the files in my skin are based on Turbine's files so you would need to contact them).
*this is unofficial reply. the site admins might be better suited to answer your question.