Difference between “kaufen”, “einkaufen”, “aufkaufen” and “ankaufen”

Question

I'd like to know:

• What's the difference?
• Which one needs akkusativ-object?

Answer 1

kaufen

Same as buy in English. Works for pretty much every situation, from chewing gum to real estate to bribery ;)

Needs an accusative object, e.g.

• Ich kaufe einen Apfel.
• Er hat ein Auto gekauft.

einkaufen

This leans more towards shopping ("einkaufen gehen" = "to go shopping"). As a noun it is also often used in the context of a corporate purchasing department ("die Einkaufsabteilung", "Sie arbeitet im Einkauf.").

Can be used with an accusative object:

• Ich habe Hundefutter eingekauft.

Note that "einkaufen" doesn't need an accusative object:

• Ich gehe heute auf dem Bauernmarkt einkaufen.

Another meaning of "einkaufen" can be to pay money to attain a certain status that is usually inherited or achieved through merit, e.g. to acquire an aristocratic title:

• Er hat sich in in den Adel eingekauft.

aufkaufen

Similar to English usage ("to buy up something").

Needs accusative object in most cases:

• Der Sammler hat alle verbleibenden Exemplare dieser Ausgabe aufgekauft.

Is also used in the context of company acquisitions. The company being bought is usually the subject and the passive form is being used:

• Skype wurde von Microsoft aufgekauft.

ankaufen

Usually used in the sense of someone offering money for certain (used) items, e.g. (dental/jewelry) gold ("Goldankauf") or used cars ("Ankauf von Gebrauchtwagen").

Needs an accusative object:

• Der Gebrauchtwagenhändler kauft gebrauchte Autos an.