It doesn't matter if the character is dead or alive, because the stores are their own objects, not dependent on any specific character. Because of this, it's possible to use two different merchants to open the same store - I did this a while back to let you access Samhan's secret stash after he's been arrested, by talking to his brother Dendis - or to access it via a placeable or anything else that is capable of firing a script. With the OpenStore function you can reduce the buy price of all items down to zero, but it does not seem to support reducing the sell price at all.
The details are a bit complex, but the short answer is if there were a way to do what you say, to move items from a store to a placeable, then you would also be able to move them from a store to another store and, as Fallen Guardian suggested, set the new one to "only sells". The advantage is you would have access to an infinite supply of whatever items of which the store had an infinite supply. But they are both hindered by one problem - I can't figure out how to grab an item from a store's inventory inside a script.