Deregister and Dequeue Stylesheets From a Plugin and Enqueue a Child’s Stylesheet/s

As kaiser noted, you probably need a higher priority.

add_action( 'wp_enqueue_scripts', 'load_alta_styles', 20 );

But if not, you can use wp_style_is to check and see after you do things if they are as expected.

e.g. you might try:

if ( wp_style_is( 'wp_listings_single', 'enqueued' ) ) {
   wp_dequeue_style( 'wp_listings_single' );
   echo '<p>Seems to have been dequeued</p>';
   if ( wp_style_is( 'wp_listings_single', 'enqueued' ) ) {
       echo '<p>But actually was not</p>';
   }
} else {
   echo '<p>wp_listings_style was not actually enqueued ... </p>';
}