The best way to limit the middleboxes to the product listing page will be to move the middlebox code from /includes/templates/your_template/common/tpl_main_page.php to /includes/templates/your_template/templates/tpl_index_product_list.php.
Delete this from tpl_main_page.php:
PHP Code:
<?php
$box_loc = '8';//middlebox group
require($template->get_template_dir('tpl_middleboxes.php',DIR_WS_TEMPLATE, $current_page_base,'templates'). '/tpl_middleboxes.php');
?>
and paste into tpl_index_product_list.php just below
<div class="centerColumn" id="indexProductList">
to get this:
PHP Code:
<div class="centerColumn" id="indexProductList">
<?php
$box_loc = '8';//middlebox group
require($template->get_template_dir('tpl_middleboxes.php',DIR_WS_TEMPLATE, $current_page_base,'templates'). '/tpl_middleboxes.php');
?>
<h1 id="productListHeading"><?php echo $breadcrumb->last(); ?></h1>
(You can put it just above
<div class="centerColumn" id="indexProductList">
if that suits your pirposes better.)
Putting it inside the listing code is tricky; if you want to do that, ask for help. I believe I helped someone else with that before.