WordPress Ajax Always returning 0

Try die() function in the end of PHP function. It will helps.

UPD: Ive made some simple version of your problem code to check it and it works. I think it will helps you.
You are able to change inner PHP script to your logic and it wont returns 0.

PHP:

function more_post_ajax(){
  echo "Hello";
  die();
}
add_action('wp_ajax_nopriv_more_post_ajax', 'more_post_ajax');
add_action('wp_ajax_more_post_ajax', 'more_post_ajax');

function custom_scripts_init(){
  wp_enqueue_script( 'custom', get_template_directory_uri().'/assets/js/custom.js', array('jquery') );
  wp_localize_script( 'custom', 'ajaxPosts', array(
    'customUrl' => admin_url( 'admin-ajax.php' ),
    'noposts' => 'No Older Posts Found',
));

}
add_action('wp_footer', 'custom_scripts_init');

JQuery:

jQuery(document).ready(function(){
jQuery("#page").append("<button type="button" class="load-more"/>");
var ppp = 2; // Post per page
var pageNumber = 1;


function load_posts(){
    pageNumber++;
    var str="&pageNumber=" + pageNumber + '&ppp=' + ppp + '&action=more_post_ajax';
    jQuery.ajax({
        type: "POST",
        dataType: "html",
        url: ajaxPosts.customUrl,
        data: str,
        success: function(data){
            console.log(data);
        },
        error : function(jqXHR, textStatus, errorThrown) {
            //$loader.html(jqXHR + " :: " + textStatus + " :: " + errorThrown);
        }

    });
    return false;
}

jQuery(".load-more").on("click",function(e){ // When btn is pressed.

   // jQuery(".load-more").attr("disabled",true); // Disable the button, temp.
    load_posts();

    e.preventDefault();

});

});