Display custom post type by custom taxonomy

The problem is in your tax_query terms parameter.

Try:

'tax_query' => array(
        array(
            'taxonomy' => 'listings_tags',
            'field' => 'slug',
            'terms' => $custom_term->slug,
        ),
 ),

And if you want posts from just basic, there’s no need for getting terms and loopin through. All you can do is WP_Query with tax_query terms parameter as 'basic'.