Anyone whose job (or hobby) involves buying and reselling something is a dealer. If you buy candy in bulk and sell it to your friends at a profit, you can call yourself a candy dealer.
Sometimes this term implies that the goods being sold are illegal or illicit. It's even more commonly used to describe people who sell art, cars, or antiques. If you play poker with your friends and it's your turn to hand out the cards, you're also a dealer. This meaning, a player who passes out the cards in a game, dates from 1600.