WordPress – show all posts + ones with specific tag

It is not advised to used query_post() as it replaces the query of the page with new instance of the query. For general post queries, use WP_Query or get_posts.

here tag__not_in takes id of the tag and not the slug, so replace sold with the id of the tag sold

like

$args = array(
    'orderby'=> 'date',
    'order' => 'DESC',
    'tag__not_in' => sold_tag_ID
);

$allposts = new WP_Query($args);