What I would do is know roughly how much I would want for the domain. A few things to think about, whether it is an active site, or you plan on using it.. did you buy it to resell later? I would tell them I am not actively looking or interested in selling the domain, but I would may consider it depending upon the offer. I would not give them any price at first but I would already know what I want to sell it for. If they come back and offer $200 for the domain, and I was originally wanting $500, I would either tell them, well I have already had an offer for $500.. or that I was looking more along the lines of $700, and see if they try to bargain with you so you do get the $500 you were wanting.