For example, let's say that each product is shipped with a unique review_id and that review_id is tied to the product. When posting reviews about said product, you need to provide a picture of the card that contains the review_id. This would need to be verified server-side before the review is accepted.
IMO this would make it too difficult for vendors to buy fake reviews at volume, since each fake review now must go through the extra step of taking a picture of the review_id card and uploading it along with the review. Since that step would take place in the analog world it would defeat most automated review posting methods.