The best you can do is make a lower offer and justify it. Go to
DN Sale Price- Domain Name Sales Price and History and do a search for the main term (usually a noun) in the domain. IE Let's say the domain you want is FastDelivery.com. 'Fast' is an adjective while 'delivery' is both a noun and the the more important keyword. Search for .com domain sales that have 'delivery' in them. Now, take the sales from the last 2-3 years and average them up. IF the average is lower than the seller's asking price for the domain, show him the comparable sales and mention that his price is over the average selling price for domains with the term you searched. As with anything with business, there are no guarantees but you might be able to get the seller to lower his price this way.