I was pretty active on Head-Fi at the time and snatched up this deal when someone posted it. I think the HD800 was still $1500 at the time.
Though that is probably not even my greatest "steal", which could be loosely seen at actually stealing. I bought the Fostex TH-900 from a relatively new online vendor, and they accidentally sent me 2 of them. I checked and legally in the US when a merchant sends you something you didn't order, you aren't required to send it back. I held onto it for a bit and that company never contacted me about it and I sold it new in box for $900. I guess they were too disorganized to notice.
I'm not all entirely lucky or wise with money. I bought the Hifiman Edition X on release for $1400 and similar tier headphones like Edition XS are less than $500 now. But I still like them enough for it to be a permanent part of my collection.
I bought a second-hand pair of Bose QC35 for like $80. Then they started having this issue where they would turn on by themselves. I Googled a bit and it turned out that it was a known issue and Bose would often replace them. So I submitted a ticket and got a brand new pair of QC45 as a replacement. Good deal! At least it was until I broke up with my GF and never got them back. Oh well, easy come easy go.