I think the lesson here for some of us that use ebay is that its not a handshake, its a legal contract based on ebay terms and aussie contract law, and if one party breaches those terms, it is very easy for the other party to walk away.
Once there has been a breach of contract, for example a buyer refusing to pay in accordance with the auction terms, there is a process where the breached party can cancel and the other is then given time to appeal. Ebay is not their mother, if they don't appeal, ebay doesn't chase after them.
If a buyer asks for the terms to be changed, that can be considered a counter offer under aussie contract law which automatically extinguishes the original contract permanently. The seller then has the right to reject the counter offer and walk away. The original contract can not be enforced.