Best US second hand dealers that ship overseas

Look up The Music Room in Erie, CO  https://tmraudio.com/

I'm not sure about the overseas but you can always contact them.

I’ve used TMR. They are excellent!

Don't you have heavy "duty" to pay, apart from shipping? Japan is a good source for a lot of esoteric hi end and there are other places that may be nearer you. I don't know if AUS has any trade relations with countries that get a "pass" on import tariffs, but that's a big factor, isn't it? 

In terms of the mechanics of shipping, it is usually encased, palletized and at best air freighted-- all of which costs money. There was a guy in NZ who was high end, he kinda went more mainstream. But my concern for you is the real cost to your door. What is it you are looking for? Some of the stuff may be gettable without buying in the States. From real dealers, with support. 

Some things are cheaper even when you include import and tax and freight. Plus, things don’t show up that often on the secondary market. Specifically, I’m looking at Ayre AX 5 Twenty, Modwright 150SE or Modwright 225KWH. Speaker wise I’m considering Marten Oscar Duos, Wilson tunetots or maybe some stenheim monitors. This is all preliminary and not ready to pull the trigger as I need to move on my Decware mystery amp,

The Music Room is a great place for used gear.   They are strict on grading if they say it has minimal signs of wear , it is most likely in really good shape.  Not sure if they ship internationally