A shop that could be used on an adventure map with restricted block availability. Weighted pressure-plate requires set amount of blocks to activate before chest can be opened safely. If the chest is opened without 'payment' then the player is attacked. The item is replenished with a dropper only if the removed item has been paid for.s
It's probably not the most elegant redstone circuitry ever designed. If anyone would like to suggest improvements, I welcome suggestions.
You will have to activate the flasher circuit (see pic).