Mary went to the box office at lunch time,but all the tickets had sold out.

sell out 意思是东西被卖完、货物脱销,主语只能用表示物品的名词充当,在英语里称作主动形式被动意思,类似的例子还有：
Your hair wants cutting.你的头发需要剪一下.（头发被修剪）
My car needs repairing.我的车需要修理.（车被修理）
This fabric washes well.这种织物很耐洗 （织物被洗）
A ship waters before sailing.船在起航前先加水.（船被加上水）
除前两个例子可以用不定式改写为被动语态,这类动词多数不能用被动语态表示:
Your hair wants to be cut.
My car needs to be repaired.