Hello,
It seems that rich results are invalid when adding comments with rating:

https://search.google.com/test/rich-results/result?id=rKDoF-n7_5I02SeNpAUh2g

Also all listings without comments/ratings are rated as 0/5 with 1 rating available. Huge bug and must be fixed ASAP!

I think root of problem is fact that theme has structured data in header as well.
Go to item.php and remove this code:
  <!-- GOOGLE RICH SNIPPETS -->
  <span itemscope itemtype="http://schema.org/Product">
    <meta itemprop="name" content="<?php echo osc_esc_html(osc_item_title()); ?>" />
    <meta itemprop="description" content="<?php echo osc_esc_html(osc_highlight(osc_item_description(), 500)); ?>" />
    <?php if(osc_count_item_resources() > 0) { ?><meta itemprop="image" content="<?php echo $resource_url; ?>" /><?php } ?>
  </span>
After this you should get valid rich data.
https://search.google.com/test/rich-results/result?id=1Mh2TTwe7OjiXH3OC8Z_-g

It seems that they are now valid but still show rating 0 with 1 rating even if there are not comments/ratings.
If rating is 0 it should remove structured data as rating 0 is not an option and it's very misleading.
Please note that the listing in the screenshot is with 0 comments/0 ratings!

https://prnt.sc/PcorcwG9HYrD

Default value should be 3.
By google, product must have rating 

If it will be 3 it will be fake. Anyways should be fixed in some way. If you ask me I would disable reviews if there aren’t any and show the reviews only if they exist