The product has been sold out.为何不用被动,没有been呢?奇怪

应该是 the product has sold out.或者the product has been sold out.都可以.
有些词是“主动表示被动的”
像：Her new book sells very well .她的书很畅销.
The coat feels soft.这衣服穿起来软绵绵的
Ice-cream goes well in summer.夏天 冰淇淋卖的很好.
以下是《朗文当代英语辞典》中二者区别：
be / have sell out:
1.if a shop sells out of sth,it has no more of that particular things left to sell （如果一家商店的货物脱销了 没有专门留下的来卖 就用 be sold out.强调“本家店”没有了,其他店或许还会有.用be sold out）
e.g.Sorry,we're sold out.
2.if a product,tickets,places at a concert sell out,they are all sold and there are none left （如果一个产品,音乐会的票,卖光了,全部卖光,“片甲不留”.强调已经没有机会得到此物了,用sth have sold out）
e.g.Wow!Those scarves sold out fast!
be sold out
e.g.Tonight's performance is completely sold out