特价：bargain price
打折：discount,bargain,price-reduction,reduced price.on sale这个词组,也不可忘
综上,可译为：bargain book market,个人以为,一个词,肩挑两担,足够了（bargain and discount book market反而臃肿）
网店,似乎用得比较多的是on-line store/shop、web shop/store,推荐前者
网店服务：on-line store/shop(2选1) service
仅供参考