The stats don't really matter to me all that much unless the NPCs are clearly antagonists that the PCs will likely end up fighting.
Depending on the game mechanics, there could be other interactions that would require stats/skills and you never know what stunts the players are going to pull on you.
What starts out as a basic "go to the store and buy something from the storekeeper" that does not require stats and skill levels suddenly takes on a greater complexity if the players decide that they'd rather haggle than pay the asking price. In the games I run, that would require at least stats and a skill check on both sides (opposed task) - with the possibility of modifiers dependent on other factors as well.
This is why I keep old character sheets.
Player: "I'm going to try to persuade him to throw in a couple of these at less than half price"
Me (Internally): "OK, his INT is... [quickly grab closest old character sheet and settle back as if I had it prepared all along] 7 and..."
Me (Externally and casually): "OK, roll on your persuasion skill..."
Depending on your players, they might just turn around and say "screw paying for this, let's just beat him up and take the stuff..." and fighting stats/skills are suddenly relevant.
Is "Storekeeper A" stronger, faster, cannier than average? Weaker, slower, more naive? Is (s)he more prone to respond favourably to an attractive member of his/her preferred sex? Is his/her proclivity in that matter obvious?
Those are the sorts of things that have very quickly become issues in my games when some wise-arse player suddenly says "I'm going to use my character's natural charm and good looks to elicit a favourable response".
Those sort of characteristics are one way - other than mere physical description - of distinguishing between "Storekeeper A" and "Storekeeper B". "Storekeeper A" is a curmudgeonly chap but is a sucker for a pretty woman. "Storekeeper B" is obsessed with the bottom line and wouldn't give his own mother a 2% discount.