With PKI, the customer reviews a product, and puts the hash of his review somewhere on the internet. The hash might be the hashed version of a JSON like so: {company: "Stuffed Animal company", product: "Stuffed Animal", review: "Very good stuffed animal toy"}
When the buyer tries to buy the product, he will verify each review against a public key, and check if all info in JSON verifies correctly. If it does not, then something fishy is going on.
The only problem is where to put the hash of the review. An official database of some kind is required, by a government or something like blockchain. That's why some people refer to blockchain as a "source of truth". It has the potential to contain information no human power can delete from the internet or modify in any way.