Online retailers often have competitive prices for books.

competitive 是形容词，来自动词 compete（竞争），加上后缀 -tive 构成形容词，意思是"有竞争力的"或"竞争性的"。 competitive prices 指"有竞争力的价格"，即价格比其他商家更优惠，能吸引顾客购买。