I occasionally do that.
I don't consider myself to be rude for doing so.
Edit: Of course there's a lot to be said about how you do it. We joke about "marble cops" because some folks can come across pretty harshly. And if the seller doesn't respond the way the person likes, it can go downhill from there. If you do that often enough for that to be how people remember you ...... well, I'm sure you don't wanna do that or you wouldn't be asking this question now.