People can change the look of a bike so easily, its hard to tell for sure what year they might be.
I met a guy who had the exact same 1996 gixer as me only his was a 99 and had changed the fairings coz he liked the color scheme better.
If i were you, i'd just go and see the bike. if its a long drive then ask for document proof in the way of rego papers or insurance details.
if he's serious on selling, he won't mind.
don't buy a bike without seeing it or riding it!
here's some good advice
http://www.clarity.net/~adam/buying-bike.html