If in doubt, let him make the first offer. If he's interested enough he's probably had some thought about it at some point in time. You on the other hand, you can sit back and relax and let him come, he wants something that you own, thus you're sitting on the more comfortable chair.
I'd post the domain for appraisal in the Appraisal Thread and you should get very usable statements. However, even though you may get opinions, i'd still let the guy that called make the first step to even gauge where he stands. If he's low balling, you can always fall back on the appraisals here. Once you have numbers, call him back, tell him that you might be intrigued to sell the domain and ask what number he has in mind, then you can take it from there.
this would be my way to go about it, others may have a different opinion, ultimately, you should proceed in a fashion that makes you feel comfortable and where you can look yourself in the mirror the next day.