Postgresql - return column of referenced table that is not primary key
You can just use a CTE for the insert, and then check the users table via SELECT:

WITH ins AS
(
INSERT INTO
    listings
    (posted_by, other_columns)
VALUES
    ('just_me', 'and that''s what I post')
RETURNING
    listings.id, posted_by
)
SELECT
    ins.id, users.internal_id
FROM
    ins 
    JOIN users ON users.id = ins.posted_by ;


dbfiddle here