I ordered an Aeron stool from Madison Seating [1] while I was setting up my home office a few months back, and the experience was pretty solid. They have good prices, and the shipping was quick and competent. I'm happy to recommend them.
...not in my experience? Herman Miller apparently sued them back in 2015 for supposedly claiming that refurbs and open-box used stuff was new, but if the complaint was founded then, it certainly isn't any more. I was never in any doubt that I was buying a refurbished chair, and for paying half what a new one would cost, the quality of the chair I got still means that as far as I'm concerned I came out ahead on the deal.