Display all posts/pages in a multiple selectbox

get_pages returns an array of post objects, so you would use foreach to iterate over each object and grab the ID and whatever else you want to use:

<?php
  if ( $pages = get_pages( $args ) {
    echo "<select multiple>";
    foreach ( $pages as $page ) {
      echo "<option value="{$page->ID}">{$page->post_title}</option>";
    }
    echo "</select>";
  }
?>