backseat buyer
A person who gets excited over other people’s future or current purchases. Usually an enthusiastic shopper for themselves as well.
Can also be used as an adjective and verb.
“I’m such a backseat buyer; I love helping my friends choose stuff as much as buying something for myself!”
“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.”