der Händler
dealer
A person or company that buys goods and resells them.
Der Gebrauchtwagenhändler machte mir ein gutes Angebot für mein altes Auto.
The used-car dealer made me a good offer for my old car.

dealer
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dealer
A person or company that buys goods and resells them.
Der Gebrauchtwagenhändler machte mir ein gutes Angebot für mein altes Auto.
The used-car dealer made me a good offer for my old car.

'Der Händler' (the dealer) is derived directly from the verb 'handeln' (to trade). A 'Händler' is therefore a person who engages in trade, that is, buys and sells goods.
Mein Onkel handelt mit seltenen Briefmarken.
My uncle trades in rare stamps.
'Händler' (dealer) is often used as the second part of compound nouns to specify the type of business. Examples include 'Autohändler' (car dealer), 'Gemüsehändler' (greengrocer/vegetable seller), or 'Kunsthändler' (art dealer).
Der Weinhändler hat mir eine gute Sorte empfohlen.
The wine dealer recommended a good variety to me.
The meaning 'card dealer' is limited to the context of card games. Words like 'Karten' (cards), 'Spiel' (game), 'Poker' (poker), or 'austeilen' (to deal) make it clear that this meaning is intended.
Wer ist in dieser Runde der Händler und teilt die Karten aus?
Who is the dealer in this round and deals the cards?
customer
'der Kunde' and 'der Händler' are on different sides in a sales situation. The Kunde buys something or uses a service. The Händler sells goods, often after buying them himself. So the contrast is clear: the Kunde pays for the goods, while the Händler offers and sells them. In a shop, at the market, or online, you often need both words together.

Auf dem Markt erklärt der Händler, woher das frische Obst kommt.
At the market, the vendor explains where the fresh fruit comes from.
Der Kunde fragt im Laden, ob es die Jacke auch in Blau gibt.
The customer asks in the shop whether the jacket is also available in blue.