Old English dælere "divider, distributor; agent, negotiator," agent noun from deal (v.). Meaning "player who passes out the cards in a game" is from c. 1600; meaning "one who deals in merchandise" is from 1610s. Illegal drug sense is recorded by 1920.
1. His heir sold the painting to the London art dealer Agnews.
2. They aim to clear every dealer from the street.