WordPress Subquery returns more than 1 row on SELECT

SELECT 
  postmeta.post_id 
FROM 
  postmeta
  INNER JOIN posts 
    ON (posts.ID = postmeta.post_id )       
WHERE 
  postmeta.meta_key = '".$core_admin_values['listing_expiration']['key']."' 
  AND posts.post_status = 'publish'
  AND postmeta.post_id = 
  (
      SELECT 
        postmeta.post_id 
      FROM 
        postmeta 
      WHERE 
        postmeta.meta_key = 'listing_status' 
        AND postmeta.meta_value != 1
  )
  AND posts.post_type = '".$core_admin_values['listing_expiration']['taxonomy']."_type'
  AND DATE(postmeta.meta_value) < DATE(NOW())";     

      SELECT 
        postmeta.post_id 
      FROM 
        postmeta 
      WHERE 
        postmeta.meta_key = 'listing_status' 
        AND postmeta.meta_value != 1

  AND postmeta.post_id in
  (
      SELECT 
        postmeta.post_id ...
  )

 $SQL = "SELECT ".$wpdb->prefix."postmeta.post_id FROM ".$wpdb->prefix."postmeta
    INNER JOIN ".$wpdb->prefix."posts ON (".$wpdb->prefix."posts.ID = ".$wpdb->prefix."postmeta.post_id )       
     WHERE ".$wpdb->prefix."postmeta.meta_key = '".$core_admin_values['listing_expiration']['key']."' 
     AND ".$wpdb->prefix."posts.post_status = 'publish'
     AND ".$wpdb->prefix."postmeta.post_id = (SELECT ".$wpdb->prefix."postmeta.post_id FROM ".$wpdb->prefix."postmeta WHERE ".$wpdb->prefix."postmeta.meta_key = 'listing_status' AND ".$wpdb->prefix."postmeta.meta_value != 1 LIMIT 1)
     AND ".$wpdb->prefix."posts.post_type = '".$core_admin_values['listing_expiration']['taxonomy']."_type'
     AND DATE(".$wpdb->prefix."postmeta.meta_value) < DATE(NOW())";     
    $expired_listings = (array)$wpdb->get_results($SQL);

ini_set('display_errors','Off');
ini_set('error_reporting', E_ALL );
define('WP_DEBUG', false);
define('WP_DEBUG_DISPLAY', false);