It would be a very valuable name for a particular industry but he is using it for a one-product business that has nothing to do with the domain name. He also has a domain name that IS the name of the product.
So, it appears that he had/bought the really valuable name and is using it merely to duplicate his web presence ...there are two identical sites.
There is no valuation tool that I would deem valid or reliable, nor is there a guidebook of any kind.
Forum evaluations of domain values - even in those forums that allow you to post specific domains - often vary wildly, even amoungst experienced members. 10-fold differences in valuations, e.g., $5K - $50K, are not uncommon. The actual dollar swings in valuation can be far greater if it's truly a name of value. To a degree domain valuation is still a "beauty is in the eye of the beholder" thing, i.e., domains are still somewhat like artworks. In this case they're viewed as brand, in addition to other measures.
In light of what you have described there may be other issues you have to be concerned with, such as duplicate content penalties.
Ask to see traffic stats, including referrers.
Sometimes it's safe to ask "What is the best offer that you have received to date?". If he/she tells you "$50,000." it might save you some time and effort if that is out of your league.
Many sellers simply wish to know if you a) have money enough; and, b) have a realistic appreciation of value. IF you have the money then, even if your initial offer gets scoffed at you can come right back to offer something more.
Wish there was a realistic yardstick for your acquisition but unless you are willing to bind yourself to the "traffic model of valuation" - which has its limitations - there is little to guide you.
You might look at the historical sales reported at DNJournal. It's possible your seller is aware of DNJ's reported numbers and could have some expectations built upon sales of analogous domains.
