a person who can find out the value of things by judging the worth, quality and condition of those things, such as the value of a house, a piece of land, a business, etc.;
The Russian Society of Appraisers (RSA) is an organisation of valuation professionals in Russia. (from Dictionary3.0)
Willis van Devanter is one of the world’s top manuscripts appraisers, i.e. he knows the quality and value of a handwritten document of some importance, usually something written by a famous person.