As soon as you put "Copyright blah blah blah" on your work, it's copywritten. You do not need to register or anything. However, registering and going through the formal process gives you greater legal protection. In this particular case, it doesn't sound like you need to go that far--you just don't want people selling your work, but I don't believe that a huge sum of money is at stake if they do.
You can read more at http://www.copyright.gov/circs/circ1.html.
Hope that helps.
-Emerson