If there is one thing I have learned the hard way it's that when it comes to doubts they won't serve you at the negotiating table and it's the person who is willing to walk away that wins at the negotiation.
So I think it's important to know in your own mind a definitive selling price, $400 might be it, but if you are desperate to sell the domain at $400 then to have any chance of getting more than $400 you have to at least be able to sell them the impression that you are willing to walk away and that they are missing an opportunity.
Chances are if they like the domain today, they are still going to like it in a couple of days time so there is nothing too detrimental with feeling them out with a higher counter offer and when they say no, say thanks but think your valuation is fair, and then just wait a day or two and then start talking with them again.
A wise fella, let's call him Neddy, once gave me some sage advice that once you give someone a counter offer you can't go up from there. So you may as well start higher and work down.