//sort order
case 'PRODUCT_LIST_SORT_ORDER':
$listing_sql .= "p.products_sort_order , pd.products_name " . ($sort_order == 'a' ? 'asc' : '');
break;
//end sort order

Updated Code (added "order by"):
//sort order
case 'PRODUCT_LIST_SORT_ORDER':
$listing_sql .= " order by p.products_sort_order , pd.products_name " . ($sort_order == 'a' ? 'asc' : '');
break;
//end sort order


More...