因为sell本身可以是不及物动词，所以本句不需要被动语态。即If something sells, it is bought by the public, usually in fairly large quantities.
和sound，look类似，都有…起来的意思。听起来，看起来，卖起来。
就是习惯用法。
The book sells well. (这里的好卖表示的是目前的状态)
