A bad seller (or buyer) could avoid negative feedback just by never leaving feedback, meaning the buyer (or seller) would never be able to publish theirs.
Parties can lobby to have the other person change the feedback, but only if they lobbied party wants to. Seems like a workable system to me and allows each to be honest without consequences like eBay's original system.
E: Also just had another idea. You could have a way to edit your feedback after the original deadline in case of fraud. If you open up a dispute like the one posted then Ebay could review the edit's on a case-by-case basis.