dealer vs stamp dealer

dealer and stamp dealer both are nouns.

Word	Noun	Adjective	Verb	Adverb
dealer	Yes	No	No	No
stamp dealer	Yes	No	No	No

As nouns, stamp dealer is a hyponym of dealer; that is, stamp dealer is a word with a more specific, narrower meaning than dealer:

dealer: someone who purchases and maintains an inventory of goods to be sold
stamp dealer: a dealer in stamps (whose customers are stamp collectors)

dealer (noun)	stamp dealer (noun)
a firm engaged in trading	a dealer in stamps (whose customers are stamp collectors)
a seller of illicit goods
the person who distributes the playing cards in a card game
someone who purchases and maintains an inventory of goods to be sold
the major party to a financial transaction at a stock exchange; buys and sells for his own account