You can use wp_query()
for this to work
$args = array(
'post_type' => 'auction',
'posts_per_page' => -1
)
$query = new WP_Query($args);
if ($query->have_posts() ) :
echo '<select>';
while ( $query->have_posts() ) : $query->the_post();
echo '<option value="' . get_the_ID() . '">' . get_the_title() . '</option>';
endwhile;
echo '</select>';
wp_reset_postdata();
endif;
Documentation for WP_Query
https://codex.wordpress.org/Class_Reference/WP_Query