I think eBay stopped letting sellers leave negative feedback because some sellers would receive negative feedback and then demand the buyer remove it or receive negative in return. Many sellers used this method as their protection from negative.
eBay have been looking into ways of helping sellers from buyers that leave negative feedback and low DSR's. I wrote a blog post on it a little while ago:
http://barry-wells.com/ebay-helping-sel ... -feedback/There's a link within the post to an eBay help page where you as a seller can report a buyer if you feel their actions are unacceptable.
For example: If a buyer buys numerous items on different occasions and leaves you neg feedback, or a low DSR. eBay rightly think that if the buyer had a gripe why would they return for more of the same service?
If a buyer continually leaves low DSR's eBay will now email them and explain the impact of low DSR's to a seller. If the buyer continues to leave low DSR's their account is suspended and If they're found to be in competiton with the seller
all of their accounts are suspended. Remember eBay log your IP address, and all accounts from that address are suspended.
So you see eBay are taking measures to level the playing field, but yes they still have some way to go.