It was then my mother remembered a toy seller had been selling toys in the place where we had been waiting in the station and that our suitcase[s] must have got mixed up.

seller 是由动词 sell（卖）加上后缀 -er 构成的名词，表示"卖东西的人"，即"卖家"或"销售者"。 suitcases 是 suitcase 的复数形式，suitcase 由 suit（套装）和 case（箱子）组成，指"旅行用的手提箱"或"行李箱"。