I have purchased two similar and have NEVER started dialogue with the domains.
I usually approach and say I have a similar website idea and if they are willing to sell the website. When they ask me if I want the domain I tell them I have a domain in mind but would be interested in theirs to forward it to mine.
Now the emphasis is on database etc and usually I can get the whole deal under 1k. If I had approached them on just the domain it would have sparked them to research it to establish value. I find on old websites the owners just kind of shrug and give up the assets as long as you don't let on that the domain was of your main interest.