An appraisal by a reputable firm offers guidance about the informed buying market, a projection of the amount that those with specialized knowledge, who routinely buy and sell domains, will be prepared to bid. It will be a range estimate. However, due to a domain's uniqueness (they're all one of a kind), domain appraisal shares in some of the same qualities as art appraisal. Specifically, there are collectors who "just have to have" a certain work of art - which will effect price - just as there are people/companies who just "really want that domain".
To find a reputable appraisal company I'd suggest you look for companies/individuals with the sales/purchase statistics AND the experience in the bargaining process. It's one thing to have access to data concerning the publicly reported domain sales (lots of people) and quite another thing to routinely participate in the process of "offer, counter-offer, sale and purchase" of domains - both reported and unreported sales.
By reputation, based upon the above criteria:
Moniker/DomainSystems
Sedo
. .
. .
Afternic
Why do you think you need an appraisal?
why?
i actually have a domain that a few people are interested in .. a couple mentioned appraisal, so i thought i'd look into it. not that i'll necessarily go with what the appraisal says its worth, because if it's lower than what i personally value at, then i won't be selling it.
It's a common scam. You pay for the appraisal and then the buyer's interest vanishes.