Aeron sizes are a bit wonky - unless you're shorter than 5'6" and ~110lbs, pretty much everyone I've ever talked to prefers the largest one. So definitely sit in it beforehand.
One of the big selling points for HM is the 10-12 year, basically no-questions-asked warranty. You won't get that on a resale, just fyi. May be worth it for the cost savings, but keep it in mind.
This is an odd one - SMELL the chair. Give it a good wiff, as I've encountered a few in offices that just straight up smell bad.
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All that said, lots and lots of start ups by these super expensive chairs and go out of business six months later, and liquidate everything. You can snag a practically brand new chair for half off or even cheaper if you're savvy and patient. Ask where they came from, though - it's not super common by any means (don't want to scare you), but knock offs do exist.
Smell the chair?
Like how bad we potentially talking here? Like 1-2 years of a sitter busting ass kind of bad?