http://www.amazon.com/Aeron-Chair-Herman-Miller-Adjustable/dp/B003M1C7XW
I'd prefer a brand new one, unless you are a very clean person and the chair is less than 6 months old.
Have you considered the Health Chair? I have been debating between an Aeron and that.
I have Prime and the Aeron appears 'eligible for Prime' if it is 'Fulfilled by Amazon' and there are a few B's that fit that description; so free two day shipping.
If you send the BTC first then I will get it ordered for you. Will use MtGox weighted average price so looking at around 131 BTC. Will also forward the tracking information and invoice. I have references if you need or we could use a trusted escrow like Kluge.