解析：buy 是短暂性动词,后面不能和表示一段时间的状语连用.
此题用while,而不用when,首先要了解一下两词的区别：
由when引导的时间状语从句,主句用过去进行时,从句应用一般过去时；如果从句和主句的动作同时发生,两句都用过去进行时的时候,多用while引导,如：
a.When the teacher came in,we were talking.
当此句改变主从句的位置时,则为：
While we were talking,the teacher came in.
b.They were singing while we were dancing.
④when和while 还可作并列连词.when表示“在那时”；while表示“而,却”,表对照关系.如：
a.The children were running to move the bag of rice when they heard the sound of a motor bike.
孩子们正要跑过去搬开那袋米,这时他们听到了摩托车的声音.
b.He is strong while his brother is weak.
他长得很结实,而他弟弟却很瘦弱.
所以,从本题来看,是强调两个动词同时发生,所以用while.