I found a GTX 670 for 275$ online, it is used and claimed to be like brand new as it has only been "used" for about 5 hours before the owner bought a 680.
Ask him about artifacting (visual spikes, discoloration, etc), and be sure there is some type of garauntee or insurance from a 3rd party. It's about $85 less than new, so it doesn't seem too good, but I don't understand why someone would "upgrade" from a 670 to a 680; there isn't a huge performance gain, and the premium is steep. It doesn't make any sense.
I know a lot of people who have done the same thing, some people absolutely will not accept a card that won't run a certain game at max settings and get well over 60 fps. For seeing if it works, tell him to make a video of the card running and him playing a few minutes of a game on it.