I think verifying the purchase is the solution If you cannot prove to me that you bought this product,you should not be allowed to influence my customers. or maybe place the non-verified purchase review in a separate category with a disclaimer pop to take these reviews with a grain of salt. it would be great if other sites like Yelp had the ability to upload a receipt.

You are correct on this. I think the industry will require MTBF testing for all components and your car must be inspected annually. Any component that is approaching the failure time must be replaced before the car is allowed on the road. Very similar to aircraft maintenance regulations.