I only buy used books.

　　我只买旧书。

　　Used还可以说second-hand,used要更常用些。

　　比如：I bought a used car and saved quite a few bucks.

　　我买了一辆二手车，省了不少钱。

　　大家一定对“二手店”挺熟悉的吧，用英语说就是second-hand shop.