if it's a lot cheaper than a new 770 then go for it. The 680 is awesome and is the same as the 770, which just has higher factory clocks, it's not next gen or anything special.
The reference cooler isn't great, so make sure it's at the lower end of what 680's go for.
It does look well used (dust on fan), but they get like that quickly in a dusty room. You'll probably have to take the cooler off and clean it, but it's not difficult.
Paypal will cover you except for return postage (which has to be signed for)
$200 would be good, $250 max. Usually though, people think it's still a recent top end card and end up bidding more than a new GTX770!
I bought (and sold) GTX680 and HD7950 s/h off ebay.