domain appraisals is an evaluation of domain name for what someone believes to be it's value, there are tools to use:
http://inventory.overture.com/ - see how many listed keyword for that domain. for example: for the word "sex" when you type it in overture it has 7124645 for the word "sex" so getting domain with "sex" on it is very valuable. also there are services such as sedo.com, sedopro.com, trafficz.com, namedrive.com to give values to your domain based on membership, next thing to do is google listings, yahoo, msn, aol listings and with the use of Search Engine Optimization tools
http://www.seochat.com/seo-tools/
still the deciding factor of what's the worth of the domain is in the hands of the owner, these are to be considered as guiding factors and not the conclutions. the more people to appraise the domain, the better it gets and the more it will be helpful to the domain seller.
HTH