As SUB0DH said, relationships should be declared in the Model, not in the Controller, you'd want to have something like this in your Listing model:
public function contact()
{
return $this->hasOne('Contact');
}
and in your Contact model you can link the contact to a listing by using
public function user()
{
return $this->belongsTo('User');
}
Be sure to read http://laravel.com/docs/eloquent#one-to-one