It's all a storm in a teacup!
An Ebay seller doesn't really have much choice in who buys the items that they are offering, they list it and someone buys it, deal done!
A buyer on the other hand has a choice who to purchase from and will probably steer clear of low feedback rated sellers.
A seller leaving negative feedback is a pretty useless exercise TBH, it doesn't make any real difference to the buyer as they can still go and purchase from whoever they like in future, and it makes no difference to other sellers as they have virtually no control over who buys from them, thus making it virtually pointless for a seller to leave negative feedback.
Surely a good seller will receive good feedback, which is important as it attracts more customers for them. Leaving negative feedback for their customers doesn't have any benefits for them, but receiving it is bad for business and so should discourage dodgy sellers.
Also, it's about time high volume sellers started paying tax!!!! i don't see why power sellers should be able to make a tax free living when they are trading as a business in all but name. I have to pay my dues, so should they.
Cough it up you bloody tight fisted tax dodgers !!!!!