I have always understood that the chain of purchase should run as follows -
(1) Buyer wins auction
(2) Buyer pays
(3) Seller leaves feedback with regard to buyer's payment
(4) Seller ships item
(5) Buyer leaves feedback with regard to item and service.
The problem is that too many sellers were not leaving feedback until the buyer had, so that when something arrived damaged/not as described/late and the buyer left a neutral or negative feedback, then the seller could retaliate.
I personally think that the new system whereby the seller can only leave positive feedback is a riduculous one and raises as many new issues as it solves (there are idiot buyers out there as well as unscrupulous sellers!). To my mind it would have been better to implement a system whereby the buyer was unable to leave any feedback until after the seller had posted theirs.
Engineers: fixing problems you didn't know you had in ways you don't understand.