While you're in the shop talking to Belethor. OH REALLY.
I wouldn't be at all against if ALL NPCs had something like 2-3 things to say to you when you aren't talking to them directly inside their place of interest (i.e. that guy in the shop) and then 2-3 things to say when you encounter them in other places.
First meeting: Hey, I work with Belethor in his shop.
Passing you on the street: Did you know we just got a new shipment in today? Come take a look. (would be great thing to say when the game actually does restock shops)
Misc.: Hello there. How are you today?
Inside Shop first meeting: Always good to see a new customer, please feel free to browse.
Inside Shop after first meeting: Finding everything alright?
Inside Shop misc.: Welcome again, good to see you.
Y'know, stuff like that.
It's rather sad that in 2011 we still have the old 90s RPG dialogue of:
"The wind feels chill tonight."
"The wind feels chill tonight."
"The wind feels chill tonight."