This is what I believe to be the most advanced Minecraft vending machine. It includes payment recognition which means you have to pay with gold or iron, anything else will be declined.
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Features:
-20 items, all with different prices
-Payment recognition, it knows what you throw in
-2 payment options, gold (worth 10 for every 5 you throw in) and silver (worth 5 for every 5 you throw in)
-7 segment display
-It knows if you can afford something
-Coin return
-It rejects anything that isn't gold or iron and sends it back to you

How it works: http://www.youtube.com/watch?v=pHcJ0xeWXd0