I tend to look at the "shopping" aspects of the game the same way as I see things in a novel or a movie. You could have a no-action, no-plot-advancement scene, but if it serves to reveal the character, then it's okay.
I'm thinking about The Empire Strikes Back*. Han and Leia are fixing the ship. Wow, sounds pretty boring to role-play, right? Well, in this case, we're seeing the two characters develop their relationship and how they feel about each other. Now that could be interesting to play.
I think the same thing goes for a shopping scene. Maybe I want to show that my PC is a money-grubbing, frugal guy, or he loves to throw around the cash to feel important, or whatever. Then there is a point to that scene, and it's worth playing out.
It's important, I think, to tell the DM/other players just what you're trying to accomplish before the scene gets started; unless you have such a good vibe going that everyone just knows what you're trying to do.
Scenes like this are even better when there is a chance for character growth. Especially through some kind of conflict.
Another point is how often do you need to do stuff like this? If I'm playing a firebrand preacher of St. Cuthbert who wants to convert everyone over to the church of the cudgel, I really only have to roleplay that scene out once a game session (or even less) to get that point across. Doing it over and over again is just going to feel tedious, like it would in a novel or movie. "Yeah, we get it already!"
But anyway. Without that character development, or without moving the plot forward, or without some kill-loot fun, that kind of "shopping" stuff is boring and I don't want to play it out at all.
* Specifically, the scene where Leia's trying to move that lever and Han gives her a hand, but she doesn't want one, and they talk about how he's a scoundrel and she doesn't like scoundrels, and they're about to kiss but C3PO comes in an ruins it for the dashing space rogue.