Kaufen and other Related Verbs

You are likely already very familiar with the verb kaufen and its antonym verkaufen:

Die Leute gehen in den Supermarkt, kaufen etwas und wissen gar nicht, woher es kommt.

People go to the supermarket, buy something and don't even know where it comes from.

Captions 12-13, Bundesregierung - Der Tomatenfisch
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Wir verkaufen Brezeln mit Schokolade.

We sell pretzels with chocolate.

Caption 20, Berlin - Judith und die „Brezel Bar“
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You probably know the verb einkaufen as well:

Fritzle geht mit seiner Oma einkaufen.

Little Fritz goes shopping with his grandma.

Caption 2, Ivana erzählt Witze - Fritzle und die Oma
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You may, however, be less familiar with the separable verb ankaufen. It is very similar to kaufen ("to buy"), but has a very specific context related to a person or a company that offers to buy items on a regular basis:

Der Juwelier kauft Gold an.
The jeweler buys gold.

Der Autohändler kauft Gebrauchtwagen an.
The car dealer buys used cars.

The separable verb aufkaufen can be used to mean "to buy up" or to buy in large quantities:

Ursprünglich hat er alte Anzüge aufgekauft, so aus den Zwanzigern und so.

Originally, he bought up old suits from the twenties and such.

Caption 6, Jonathan Johnson - Herr von Eden
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The verb aufkaufen is also used when a company takes over or acquires another company:

Facebook und Amazon wurden von Google aufgekauft.
Facebook and Amazon were acquired by Google.

Please note that the above statement is purely hypothetical and is in no way intended to represent the intention of any companies, real or imagined!

Further Learning
Read this excellent article on the four verbs above, and see this list of words that use the root kaufen. You may then go to Yabla German and find examples of the words used in a real world context.