After building:
-Fill the hopper above the chest with the 22 of item you are buying ("money") (Make sure all the inventory slots are filled. I reccomend putting 18 in the first slot, and 1 in the rest.)
-Fill the dropper with the item you are selling

To use:
Throw the "money" item into the hopper (under the repeater)
The item that is being sold will come out of the dropper


Materials needed:
3 blocks
1 chest (or other storage block)
1 comparator
1 dropper
2 hoppers
2 redstone dust
1 redstone torch
1 repeater

optional:
1 more comparator
2 more blocks
1 more redstone torch
1 carpet
1 painting
1 stair