I bought a 1996 R400 (same frame) in late '97 and have put (about) 17,000 kms (in the sunshine only) on it and it's like it's still new!

My previous bike (steel frame) went 53,000 kms before I replaced it.

Seems to me the 2 questions to consider, is the frame in great shape and how much $?

If the frame is damaged, it's worth squat, but if it's good but the owner want's almost what he paid for it. get a new one instead. If he's offering half price, go for it!