Backseat buyer is a person who gets excited over other people's future or current purchases. Backseat buyer enjoys helping other person choose stuff as much as buying something for himself/herself. Usually an enthusiastic shopper for themselves as well. It can also be used as an adjective and verb.

“后座购物者”会因为他人即将要购买或现在要购买东西而感到兴奋。他们喜欢帮助别人挑选东西，对他们而言，帮别人购物和给自己购物一样快乐。他们自己通常也是狂热的购物者。“后座购物者”的动词形式是“后座购物”，即backseat buying。

Example:

Whenever I think about getting a new gadget, my father goes backseat buyer.

每当我想买一个新玩意儿，我爸总是要做我的“后座购物者”。

If you don't stop backseat buying right now I'm going to smack you.

如果你再不马上停止“后座购物”，我就要给你一巴掌了。