This is the 10364th most frequent Korean word.
구매했던
"Had purchased."
The term '구매했던' is used here to describe an item ('책' - book) that the speaker purchased in the past.
제가 구매했던 책을 친구에게 빌려줬어요.
I lent the book I had purchased to a friend.
In this sentence, '구매했던' modifies '가방' - bag, indicating it was bought by '그' (he).
그가 구매했던 가방은 정말 멋져 보였어요.
The bag he had purchased looked really stylish.
Here, '구매했던' relates to '자동차' - car, indicating a past purchase made by '우리가' (we).