を買ったの

bought …?

→ を
phrase Common #3,178
bought …? を marks what was bought, and の often gives the sentence a casual questioning tone.
JA: その本を買ったの?
EN: Did you buy that book?

いつその帽子を買ったの?

When did you buy that hat?

とにかく金魚を買ったの

Anyway, I bought a goldfish.

— 恋する惑星 (1994)

どんな本を買ったの?

What kind of book did you buy?

いつその時計を買ったの。

When did you buy that watch?

トムにあげるためにセーターを買ったの。

I bought a sweater to give to Tom.

トムはどこでこれを買ったの?

Where did Tom buy this?

いつコンサートのチケットを買ったの?

When did you buy the concert tickets?

何でその帽子を買ったの?

Why did you buy that hat?

何でまたこんな高い辞書を買ったの?

Why did you buy such an expensive dictionary again?

トムにあげようと思ってセーターを買ったのよ。

I bought a sweater because I thought I’d give it to Tom.

トムって、オレンジジュースを買ったの?

Did Tom buy orange juice?