I just purchased an Aeron a few days ago. I paid $380 on Craig's List, which you can see is a pretty good deal. It took some waiting. Most of the used Aeron chairs are done by used furniture resellers. These chairs are generally in very poor condition because they were used and abused by office workers. It's much better to get the chair from a private seller since they most likely treated it as their own investment. It's not that hard to tell either. Private sellers are far more responsive, sensitive and have better customer service in a sense. They treat you like a real person. People resell expensive chairs for a living, whether they are a used-furniture dealer or not, generally list a large quantity of the same chair and reply in short, pithy sentences. In the end, they don't care too much if you end up buying or not because they have so much other inventory that another customer will come along. I still don't understand that mentality.
Note that you do not get the famed 12-year warranty if you purchase it used. Only the original buyer receives this. For a new fully-adjustable Aeron at $850, I'd expect to get at least 50% off if I was taking this risk. Herman Miller also does a great job at part replacement so in the rare case you do have a problem, you can pay a fee to have it fixed.
The older models, which you generally find on Craig's List, aren't much different. The only difference I've noticed is the arm height adjustment is more difficult since you have to turn a wheel several times to loosen or tighten instead of just using a lever present in the new models.
Also, used-furniture retail stores are generally way overpriced. The average I saw was $600. The extra markup is of course to pay for the overhead of the brick and mortar store.
I have so much knowledge because over the past month, I watched Craig's List like a hawk for Aeron chairs, and visited nearly every used furniture store in the Phoenix metro area. I've sat in dozens and became in expert in Hermal Miller chairs. I corrected furniture store dealers on how adjustments work, and what "new" pricing really is. I did this all and only saved $400. I hope my time and effort helps you though!