Here is a coustamisable shop. It is an adaptation to Minecraft with dummies design (https://www.youtube.com/watch?v=3bDUtJd9B5Y) to allow for more than 3 items at a time.

How to use:
1. Add the required item to the dropper, and it will be sucked in.
2. Press the button and wait for ALL items to dispence.
3. Carry on pressing the button untill no more items dispence.
4. If the redstone lamp is off, the shop is out of stock

How to coustamise
The items in the back hopper timer counts how many itms to dispence at a ration of 2n-1 (where n=number of items in the hopper).
The items in the double chest are the items that are dispenced.

In the scematic
The input is set to redstone compariters
The output is 9 redstone blocks