How to check all items in a loop have post thumbnail?

Before you do your output, do something like this:

$no_thumbnails = true;
foreach( $shows_sales as $p ) {
    if( has_post_thumbnail( $p->ID ) )
        $no_thumbnails = false;
}

Then in your loop you can do something like this:

<?php if( $no_thumbnails === true ) : ?>
    <!-- Your output here -->
<?php endif; ?>