In previous ES games you were able to invest your gold in a business and would in turn earn profits, but what if you could do this in ESO?
You would be able to invest in a particular NPC store if you play your cards right.

It would almost be like stocks and depending on what the player community suddenly start buying, you could win or lose gold.

Any thoughts?

seems to me this is something that could easily be exploited

I like this idea (only it needs to be done so it won't be exploitable).

Perhaps after completing certain quests for shopkeepers they can give lower prices or increase the vendor's buying power so you can sell them more expensive gear.

Another option is that the merchant does not entirely trust you and charges you more for your initial purchases until you have established a good "credit" line with him/her.

Yeah, it would be good to interact with vendors on a more intruiging level and not just a simple buy and sell basis. LOL like in Oblivion where you followed that merchant and found out he was buy merchandise from a corpse pillager/ grave robber.