Transitive verb and Godan verb
かう
トイレットペーパーと肉とやさいとしょうゆを同じ日に買いたいです。
I want to buy toilet paper, meat, vegetables, and soy sauce on the same day.
どうしてそのスマホを買うことにしたんですか？
Why did you end up buying the smartphone?
コウイチのボブルヘッド人形を買うために、長い行列ができていた。
There was a long line of people waiting to buy the Koichi bobblehead.
This word consists of kanji with hiragana attached. Because the hiragana ends with an う sound, you know this word is a verb. The kanji itself means buy so the verb form is to buy.
Since this word consists of a kanji with hiragana attached, you can bet that it will use the kun'yomi reading. If you remember your kanji readings, you'll be just fine.