eBay is a good tool for determining value, search for your item selecting closed auctions and you often get a list of your item, bike in this case that has already sold (or not). The price of those transactions helps me determine relative value.
Likewise craigslist shows you another view of what people are asking. If your target bike is being listed for less or more than your deal, you have more information. Sometimes you have to search outside your area to get a few hits, especially if your target bike is popular and scarce.
Some other things to consider, is the person a 'dealer or private person who sells a lot of bikes'? Or, is it a person selling their own or a friend's bike? If possible, take the bike to your bike shop/mechanic and have them look it over. Just because it shifts right today does not mean the drive train is not worn out. Example, my Trek Nav looks like new, rides like a dream, but has 1500 miles on it and I'm pretty sure it needs a chain, cassette and who knows what else.