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.
oDesk gets around this by having a time limit on when feedback can be left. If one party fails to enter feedback, then it posts the other's feedback anyway.
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.
Then you place a time limit on submitting feedback. Say you have 1 week-1 month. Still not perfect since it sounds like in the posted scam he didn't find out for over a month, but it might help in many cases.
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.
Online sites like eBay, always want to reduce their personnel costs. Dispute resolution should be a last ditch effort that exists to identify loopholes in a system that should ideally NEVER need dispute resolution.
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.