I rarely even look at the seller's rating. I buy off eBay probably 100 times a year or so (twice a week on average) and have since the first year they were in business, and I've never been scammed even a little. I just went and checked and over half of the people I've bought from in the last year have been < 99.5%, one down around 98%.

Just looking at the ratings, it looks to me like a lower rating is mainly indicative of someone selling the sort of stuff that idiots buy (often used) with high hopes and expectations. They buy something "as is" then rate the seller low when they get it and it doesn't work, or they buy something clearly not suited to their purpose and gripe that they didn't read the description. I see the same thing on Amazon ratings - one star because the idiot is complaining that something that clearly says in the description "not Mac compatible" turned out to be not Mac compatible. Or they rate an item one star because UPS ran over it.