If you are searching for specific seating, such as front row or box office seating, they may not be available off the original ticket source, even when the event isn't sold outside.
Although this article can be read by itself, can be suggested may read the prior article on this subject foremost. http://www.pinnaclebattleship.com/wiki/index.php/Broadway_Tickets_For_Less:_Top_Tips_To_Save_Big