I think the reviews for products are gamed but I don't think the seller feedback ratings are.
I used to sell used books as a Amazon marketplace seller and I once sold a book that was damaged and put that condition in the description. The buyer bought and gave me 1 star rating for it and described the damage that I already put in the description. The buyer just didn't bother to read the description before he bought it. I couldn't get Amazon to remove that unfair rating.
If there's a way to game the seller feedback in a sustainable way across thousands of ratings, I'd like to know how. Sure, sellers could offer refunds to bribe customers to change their seller rating from 1 star to 5 stars (not sure if that's even possible) but eventually, the seller will run out of money with that scheme. On the other hand, product reviews are easier to hack.
Maybe I'm just extremely cynical, but you don't feel those are somehow gamed? For parts that you've described, I've actually moved to eBay as my first stop.