I've bought almost all of them on this forum, so I wouldn't want to say specifically.
It would probably have to be the freaking MINT, new in box that had never been opened K340s that I later sold to a scammer in SE Asia -- so I was out the K340s AND had to refund the jackass his money. (this took place on eBay, but he contacted me thru head-fi when I expressed concern... the one time I've been fooled online)....
It was too small an amount to bitch about though...
Until I saw what 2 pairs of NOS k340s recently went for in the FS forum... ARGH
Now to avoid confusion -- I had ANOTHER pair of nearly-new K340s that I sold to a very upstanding member on head-fi who happens to live in Asia. If you're reading this -- PLEASE realize I'm not talking about you!!
So the 2 k340s I have now are the 4th and 5th pairs I've owned, but I went through 3 mint or near mint sets to get them, so it's not a very happy memory.
Lesson learned: freaking HOLD ON to headphones. I didn't need the money when I sold them, and they've gone up in value since then.
To top it off, K340s are very difficult to amp, have a built-in terminate date because of the drivers, and are the heaviest headphones I own. But I'm NOT getting rid of them this time.