I'm attempting to make a catalog page but simply showing the posts of the specific custom type.

Loop begins such as this:

 <?php while ( have_posts() ) : the_post() ?>

How do i customize the code simply to show posts from the custom type known as "classifieds"?

Thanks!

You would be much best creating your personal query, by-passing the publish type check altogether

query_posts('post_type=my_post_type');
while (have_posts()):
    ...
endwhile;