I would be worried about two things:
1. Is the traffic real. Seems like way to often when you buy a domain for its traffic it mysteriously drops off after the sale.
2. Is the domain going to get a C&D. If so, why buy someone elses trouble and headache.
As far as the value, that is hard to say without knowing what keywords would work best for the site.
Also, some people think 6 months of revenue is fair, where others feel that 1-2 years of revenue is a fair price.
Anyway, best of luck to you.