der Kauf
purchase
The process of acquiring something in exchange for payment; the act of buying.
Der Kauf des Hauses muss notariell beglaubigt werden.
The purchase of the house must be notarized.

purchase
Loading your saved state.
purchase
The process of acquiring something in exchange for payment; the act of buying.
Der Kauf des Hauses muss notariell beglaubigt werden.
The purchase of the house must be notarized.

'Der Kauf' (the purchase) is the noun corresponding to the verb 'kaufen' (to buy). It describes either the act of buying or the thing that was bought.
Wir müssen den Kauf der neuen Waschmaschine gut planen.
We need to plan the purchase of the new washing machine carefully.
To say what is being bought, German often uses the genitive or the preposition 'von' with the dative. The genitive is more formal.
Der Kauf des alten Hauses war eine große Entscheidung.
The purchase of the old house was a big decision.
When 'der Kauf' (the purchase) refers to the thing that was bought, it is often used with evaluative adjectives such as 'ein guter' (a good), 'ein schlechter' (a bad), 'ein teurer' (an expensive), or 'ein günstiger Kauf' (a cheap/good-value purchase).
Das Fahrrad für nur 100 Euro war ein wirklich günstiger Kauf.
The bike for only 100 euros was a really good-value purchase.
The expression 'etwas in Kauf nehmen' means to consciously accept a disadvantage in order to achieve a goal. The disadvantage that is accepted is in the accusative.
Um am Meer zu wohnen, nehme ich die hohe Miete in Kauf.
In order to live by the sea, I accept the high rent.
sale / sales
'der Kauf' and 'der Verkauf' belong to the same situation involving money and goods, but they show two opposite sides. In a 'Kauf', someone receives something and pays for it. In a 'Verkauf', someone gives something away and receives money for it. They are easy to confuse because it is often the same action, just from a different point of view. Memory tip: The customer talks about 'Kauf', while the dealer or owner talks about 'Verkauf'.

Vor dem Kauf des Fahrrads prüft Anna noch einmal die Bremsen.
Before buying the bicycle, Anna checks the brakes one more time.
Für den Verkauf des alten Sofas macht Max Fotos und stellt sie ins Internet.
To sell the old sofa, Max takes photos and posts them online.