I think this is a pretty good idea, it wouldn't harm anything or anyone.
I don't think that it would directly affect the economy considering everyone would have the ability to buy an interior change, eventually it would balance out and other interiors wouldn't be worth more than the other; it's just personal preference.
I think it's quite unrealistic for all houses to have the same interior, it doesn't make any sense. By adding something like this to the store would bring some variety to how houses look on the inside, exactly how it's supposed to be. I've never seen houses in real life which look identical to another one (maybe the square feet or shape, but not the interior design and so forth).