You must use
$list_obj->prepare_items()
before $list_obj->search_box('Search', 'search')
function!
Example:
<form method="post">
<input type="hidden" name="page" value="<?php echo $_REQUEST['page']; ?>" />
<?php $this->sales_obj->prepare_items();
$this->sales_obj->search_box('Search', 'search');
$this->sales_obj->display(); ?>
</form>