There are these services that for anywhere from $5 to $20 claim to give you an appraisal of a your domain name. Worthwhile? Just a rip off? Is any service better than the others?
What domain name are you trying to sell? Seems like it'd be easier just to put it up for sale and see what type of offers you'd get. Seems like there is no way that you could really appraise a domain name...
Realistically, a domain name is worth as much as someone is willing to pay for it. An appraisal might give you a nice theoretical value, but it's not really going to give you anything useful if you're seriously considering selling it.
I should have added - there's not an active enough market for specific domain names for there to be any kind of credible appraisal value, unless its super-generic like business.com or games.com. That's why I don't think an appraisal will net you anything of value.