Between $100-200 less than you purchased it for.. Having Apple Care with it helps the sale. If Apple has refurbs or used in stock then drop it $250-300 lower.
How much could I reasonably expect to get on resale of an rMBP this October? Specs would be 2.7GHz, 16GB RAM, 512Gb SSD.
I would be shipping it only inside Australia.
Why are you selling your 2 month old BRAND NEW High End MacBook Pro Retina? In October it will be only 4 months old!