There is a typo in the classifieds plugin. It is located on line 225 in the file:
Currently, line 225 is written as:
<?php echo do_shortcode('[custom_fields_input" style="editfield"]'); ?>
It should be changed to
<?php echo do_shortcode('[custom_fields_input style="editfield"]'); ?>
The extra quotation mark is causing the shortcode not to process. This shortcode is the one that allows the user to set duration and cost for the ad. This typo is present in version 18.104.22.168 of the Classifieds plugin, which I believe is the currently available version.