I only buy used books.
我只買舊書。
Used還可以說second-hand,used要更常用些。
比如:I bought a used car and saved quite a few bucks.
我買了一輛二手車,省了不少錢。
大家一定對(duì)“二手店”挺熟悉的吧,用英語說就是second-hand shop.