Thank you for the files, this clears things up a bit. Here is how I see things.
Currently you are having users login with the USERS_LISTINGS_INPUT_DATA_LogIn.php page. In this authenticate user server behavior you are storing the userID in a session variable called USER_ID. This is the session variable you should use to insert the record.
Rite now on the listing insert page you are setting the same session variable to itself but only if it is not set. This means that if the user is not logged in when they get to this page the USER_ID session variable will always be blank.
To resolve this you need to add restriction to the listing insert page to force users to login before they can insert a listing. This will ensure that the USER_ID session variable is set when they are on this page, and if not they will be redirected to the login page.