Hey mate,
you may already be aware of this but overclockers.com.au have pretty active trading forums. Mainly for PC but I've seen a lot of older stuff such as old consoles, arcade machines, and Amiga stuff get bought and sold from time to time. If you throw up a 'wanted to buy' thread asking for Amiga stuff you'll probably get a few bites.
Unfortunately there are minimum time and post limits before you can use the trading forums... I think you gotta wait 3 months after signing up. Pain in the arse but its worth signing up just so you can use the trading forums in the future.
I take it you've already tried amibay.com?
