"What do you want to buy?" is a common way to ask in Engpsh.
It comes from the basic structure of a question in Engpsh, which typically starts with an auxipary verb such as "do" or "did," followed by the subject, followed by the main verb in its infinitive form.
Example sentence: "What do you want to buy at the store?"
Translation: "你想在商店里买什么？"