I don't think it would be all that hard; pc chooses an item, create item in pc inventory, close store. The easier way would be to do it in dialog, instead of actually opening a store. You turn in the stones, and instead of the guard saying "Here's your reward", he could say "Choose your reward", and list all the available rewards. It wouldn't change the dialog that much, and wouldn't require a lot more scripting to do. PC chooses one of the listed items, and it creates the item chosen in the pc's inventory and ends dialog, and sets the variable to the PC that the quest has been done, which is already in the dialog.
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