The 鑑定士【かんていし】 I have in mind is the one that this an expert about a type of goods (e.g. brand bags, diamonds, luxury watches), and is able to recognize and value them. He often use his knowledge in pawn shops or in auctions of such goods.
The term "expert" is too vague. The term "appraiser" seems very close but a bit old-fashioned to me, but I am not native in English so I may be wrong.
Which verb would you associate with this work (鑑定【かんてい】する)?
I know this is not a translation service, but I believe the term is technical enough to deserve a question.