Feedback on eBay is about the transaction not the product.
If a product is Not As Described, eBay (and Paypal) have very strong Buyer Protection policies, with a 45 day deadline for complaints.
If you know the product, you can use your Watch List to compare up to four listings at a time for price, quantity and shipping cost.
Look at the seller's feedback. Any percentage under 99% is weak and under 98% is a bad seller.
You can also look at the negative and neutral feedbacks by clicking on the number. This can give you a picture of the seller's weaknesses. And some hilarious invective.
Then look at the Detailed Seller Ratings, for a picture of how the seller treated her customer.