Unless the domain owner is desperate for cash (i.e. a motivated seller) it will be hard to knock the price down. Firstly by expressing an interest the current domain owner now knows that the domain has some value. And then there's nothing to stop the domain owner from raising the price since you're interested. This isn't like buying a used car since the domain is unique so the idea that you can value a domain and use that to bargain doesn't hold up.