Here's a breakdown:
* Kaufen means "to buy".
* Verkaufen means "to sell".
Let's look at some examples:
* Ich kaufe ein neues Auto. (I am buying a new car.)
* Der Verkäufer verkauft frische Früchte. (The seller is selling fresh fruit.)
You can think of the two words as opposites: buying something is the opposite of selling something.