Currently, shop signs are only functional when attached to the side of a container Old shops were able to be on the block above the container What would be cool is if the shop signs were not attached to the side of a container, they would search the y-axis for the closest container so that shops like this would be allowed
An alternative could be to add the ability to assign containers to shop signs, so you could have multiple containers associated with a single sign. You could store your bulk cobblestone in 40 double chests and allow people to buy that cobblestone using one shop sign.
to be fair it would be nice to have your chests full of stock under your island and just having the buyshop for the item on a wall