I'm assuming you're talking GBP? Where I live a new 690 goes for ~750 GBP, so for me this
would be ~30% off. If the card works and you can trust the seller I think this does not sound too
bad.
One thing to consider though: With the 700's rumoured to be around the corner, maybe the
600 series would fall in price on the second-hand market very soon? Or you could get an equivalent
700 card for less? I'm not sure though, I remember the 500's didn't really fall that much for quite a
while on eBay and similar sites after the 600's had come out, and you can always play the
waiting game with PC components.
But as things stand now, everything else being ok, this doesn't sound too bad (at least to my
ears), I would at least consider it (not knowing all the remaining details of the deal).