Unmanned shops (which are called Mujin Hanbaisho [無人販売所]) can mostly be found in rural areas where it doesn't make too much sense to have somebody around watching over the goods all the time. This search query shows you more of these unmanned shops - most of them selling veggies.
At these unmanned shops, there is usually a box where folks can slot money into. The awesome thing about this shop however is not only is it unmanned, but the money is actually placed in a box like this so that folks can take change too - behind the cardboard at the bottom of the tin is a load of notes which amount to over 10,000 yen.
If there was just a box with a slot then folks need to have exact change before they bought something.
The honesty of folks over here is something I just love about Japan. Its not like we don't have folks stealing stuff over here - its just that the rate is much lower than what I've experienced in other parts of the world. I'm wondering if money left out in the open at a shop like this would/does work where you are?