I would let him/her know you are interested but did a lil homework on the age/original price...
Mention bicycle blue book....heck, print it and take it with you...
Then ask if they are open to an offer... which should be $200-$250 ($250 if it's pristine and needs nothing...) Really check it out and take at least a 5 min test ride where you shift through all gears a few times, brake in combos, jump a couple curbs, check the frame for damage, crank it tight, spin the wheels for tru... etc

Good luck!

19" if fairly tall, so in general, you should be about 5'10".... do not buy the bike if it's too tall or short! Don't fall in love with how it looks, fall in love with how it fits and rides....
Lock in a time to see it, and swing by a local bike shop (preferable that sells Kona) and check out a 19" with a sales person...
I hear so many stories of wanna be riders that buy the wrong size, have best intentions, but when they ride but it's not comfortable (even subcontiously) the new bike collects dust.
lol, I did it recently with a beautiful old light cannondale mtb... but it was too tall and I sold it....