Eames walnut stool  

If I were seriously interested in the item and in Australia, I would thoroughly examine everything available to see related to the possible sale (listing, feedback, history etc.), contact the seller, talk about it, see how I felt, and take it from there.

Here's a good old DA thread that might help you out:
http://www.designaddict.com/forum/General-discussion/Eames-Walnut-Stools
Never say never, but I would be very surprised if someone was making knockoffs that looked as good as the listed item. If so, then it will be very difficult to tell from here on out.
I don't believe Eames stools ever had Herman Miller marks on them.

Thanks for the comments so far, does anyone else have an opinion on this?