Hi,
Your problem is different from @41global.
It's normal that these URLs don't return 404 errors.
However, these URLs are only referenced if you don't configure the "canonical URL" setting of your products (which you should do) with the main URL you want to be indexed for your products.
For example, on our website marketplace, you can find that product page:
www.hikashop.com/marketplace/product/229-adyen-sha-256.html
But you can change the id of that URL to any other product on our website and it will display the product page of that product even if it's not supposed to be in the marketplace.
However, since each product has its canonical URL filled in with the URL we want to be used, only the URLs we want get indexed by search engines.