This is my Dropper+Hopper redstone system I use to make shops. I wont go into details about how it works here but there is a readme book when you install the map.

One of the maps I used this for is my Mob Slayer Arena: www.planetminecraft.com/project/mob-slayer-arena-mini-game/

This idea is completely original as I have never seen anyone use it before. If you use this idea in your map be sure to give me some cred and a link so I can check it out.