My 0.02 here:
I've been saying this very thing for years now. In fact, it's why I pulled my resources from the WHMCS marketplace. It's far too easy for someone to just create a false review, claiming "this is bad product" when they haven't even tried the product / services.
In my own experience, I manually check everything against blacklists for fraud, abuse, etc. In every single case, these reviews come from people who are upset that they were denied service, because of that. They're not 'legitimate' reviews. They're individuals simply trying to attack developers (likely competitors, even).
The product owner has no recourse here, none. This is not how you run a proper marketplace, at all.
Again, just my 0.02