Refurbished Headphones

Buying used headphones from a reputable store with a good return policy is almost required. Its easy to damage headphones and for them to still sound "ok". Especially if the person never heard what they should sound like. The problem with high end headphones are no 2 sound exactly the same, Ive read a couple reviews of headphones from headfi, like the Sennheiser Hd800, where people will return/exchange brand new headphones because they had sharper highs than the last pair of HD800s they borrowed from their friend. On the other side some high end headphones sound better used as they need a longer burn in, and will sound much more "warm".
In short, if it has a good warranty, exchange, or return policy why not try it out. I usually just go for new when it comes to >$300 headphones as the mark down for being used is minimal. Id check out the market at Headfi. These are people who generally care about their headphones and wont send you a broken,blown, distorted pair.
Good luck.

Ps. I would never buy a used computer or tablet. There are too many ways to damage a computer without it being visible or immediately obvious. (water, shock, overheating, bad soldering etc.)