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.
Whenever I think about getting a new gadget, my father goes backseat buyer.
If you don't stop backseat buying right now I'm going to smack you.