How can I display all post IDs from the taxonomy?

Just some code to get started. This will get you all the IDs for job_listings that are assigned to term 4 in your taxonomy.

<?php
    $posts = get_posts( array(
        'posts_per_page' => -1,
        'fields' => 'ids',
        'post_type' => 'job_listing',
        'tax_query' => array(
            array(
                'taxonomy' => 'agency',
                'field' => 'term_id',
                'terms' => 4
            )
        )
    ) );

    echo implode( ', ', $posts );
?>