It's easy to set up a redstone lamp and comparator for a chest, so that the lamp turns on if the chest contains any items. When all items are removed by the player or drained by a hopper, the light turns off. However, I tried to do this with a chest shop, and the signal does not update when removing all items by clicking the Buy sign. Instead, the signal only updates when I manually open the chest. If this was fixed, it would be very easy to build chest shops that have lights on all the chests showing whether or not stuff is in stock!
Interesting idea. I've always thought about a shop that, when a chest is full, it triggers pistons to close the area between the player and the chest so the player can't see the chest anymore.
