The problem is simple, sellers can hold the buyer to ransom, with the threat of negative feedback if the buyer dares to complain. Many times when Iv bought an item, I honestly considered leaving either neutral or negative feedback because the item was in worst condition that stated, or postage was a rip off for the cost (I've paid £10 delivery, and the stamps show it cost £3) or the seller was slow in posting.
I've also had many sellers email me saying leave postive feedback and they will do the same to me. Then I sent them an email back saying they should leave feedback first. This is one issue I wish ebay would address, its such a simple issue to solve.
When buying an item, its the buyers job to pay for the item, once thats done their role in the matter is done. The seller should then be forced to give feedback for the buyers part and ONLY the buyers part (Ie: feedback on if they paid quickly).
Then when the item arrives, its the buyers turn to give feedback on the item, without fear of negative feedback in return. If at this point the seller hasnt already left feedback then they forfit the right to leave feedback as soon as the buyer has left them feedback to stop them fighting back with feedback. (It takes at least a day or two from payment till item arriving in post so they had plenty of time).
This may sound against the seller but its really not. I sell a lot of stuff on eBay and as a seller I have had no problem giving feedback the moment the payment is clear. If a buyer then leaves me negative feedback you have the system there to challenge it.
I got one negative feedback removed that someone left me for the item getting lost in the post. The user choose to pay less and get the item sent unrecorded, so I had to wait 28 days till I could claim from Royal Mail. After 7 days I offered a refund to the user then planned to wait 3 more weeks before I could get the money back from the postal company. I explained this to eBay on the Dispute console, and they went in my favour saying I did everything right and the negative feedback was removed. So the system does work.
Then again eBay is too big and too scared to upset the big sellers (who make them a lot of money) who love using negative feedback threat to stop getting negative themselfs. eBay needs a major overhall, or proper competetion so it can catch up with common sense.