I remember seeing videos before oblivion came out were npcs would go into shops in towns and buy from the vendor. Obviously they took this out, but i was wondering if there is a mod were they have random npcs that will go into shops and buy the items.

There's no mods I know of that have NPCs actually go into the store and shop, but "Enhanced Economy" will mimic NPCs shopping by having the goods, and amount of gold, available to a vendor fluctuate throughout the day.
Additionally, "Trade and Commerce" allows you to open your own store and sell your goods. I haven't run Oblivion with it yet, but it may have NPCs shop at your store.

Sadly, no npc's ever set foot inside your store (other than your hired help). Your goods simply disappear in the middle of the night to be replaced by gold in your coffers. Would be nice though.