Used i>clickers are great just as long as the cost of the clicker plus new batteries together equal less than a new one.
Clickers use 3 AAA batteries.
DO NOT use Duracell batteries as they are shorter than other batteries and don't work. A fresh set of batteries should last around 200hrs.
For the scoop on sharing clickers, how to register a clicker & what to do if the ID rubs off see the Bookstore's
i>clicker FAQ pages.