I'm a seller on another account, as well as a buyer, so here's my opinion (for what it's worth):
Most of what I buy is bulk lots of postage stamps, both mint and used.
Each lot is different, so how can my rating of the product itself help future buyers, especially if the rating of the product is attached to similar products from other sellers.
It's a bit different when buying new things, but even then it really depends if the product rating is lumped in with other similar products, or if the item has to be exactly the same. eg. do they compare Kmart clothing to designer brands, or $2 shop gadgets to brand name gadgets.
I don't really think that the product rating is necessary as part of the feedback process, but don't have a problem with it being there, as long as it's voluntary, not mandatory, as it can be annoying as a buyer when all I want to do is give feedback to a good seller.
