dealer vs seller

dealer and seller both are nouns.

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

As nouns, seller is a hypernym of dealer; that is, seller is a word with a broader meaning than dealer:

Other hypernyms of dealer include marketer, trafficker, vender, vendor.

dealer (noun)	seller (noun)
a firm engaged in trading	someone who promotes or exchanges goods or services for money
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