If Bethesda gets around to letting modelers input their own work or non-Bethesda existing models into Oblivion, this mod would probably be possible. There is even a rudimentary cart (the mining cart) from Morrowind that could be used - it isn't fancy but it is a cart.
A scripter could attach the Moveable Container script to the cart probably, and then it would be able to move and hold an inventory of items for you like a chest or closet do. It would probably also have to be scripted to move in unison behind the horse. It's weight could be adjusted to nil or zero so it didn't emcumber the horse. But the larger the it's weight - the more inventory items it'll hold. If it works like Morrowind anyway.
I think there were also Guar carts in MW, weren't there? And if it is ever done, I'm sure modelers would want to make their own variety of horsecarts. As for selling from the cart, the simplest way I know of would be to have a merchant who goes along with the cart and assign ownership of the cart/container to him and then lock it and you would have to buy from him to get the items in the cart.
I like this idea, we'll see if it happens.
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