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

@katalin2k
You can do it.
Then another day you come back asking why you have error on google related to missing aggregateRating and we should fix that

Right now it looks like this if no reviews have been added:
  "aggregateRating": {
    "@type": "AggregateRating",
    "ratingValue": "0",
    "bestRating": "5",
    "worstRating": "0",
    "ratingCount": "1",
    "reviewCount": "1"
  }

Isn't it more correct like this?
{
  "aggregateRating": {
    "@type": "AggregateRating",
    "bestRating": "5",
    "worstRating": "0",
    "reviewCount": "0"
  }
}

Or maybe  worstRating should be 1 because you should not be able to rate as 0 if you ask me.

Such feedback should go to google
For me ratings should not exists if your product does not have ratings, but google does not think so.