Yes, gaming the system is pretty much inevitable with such a thing. I noticed you took inspiration from ATM machines, well, why not take it a step further.

I know of many schools, high schools to be specific, that use a sort of student money account for purchases at canteens. Instead of dispensing the cash, students will put in their ID and add the funds to their account. 

This allows you to add another check to the system similar to how ATMs handle cheques. When a product is deposited in the system, it is placed in a container marked with the product, amount worth, date, and student. The funds are immediately added to the student's account. At the end of the day/week/whatever, a human inspector then verifies the packages to determine if they meet the standards required (are sealed, contain the product, etc.). 

If there is a problem, the student's account is charged the amount and they are sent a correspondence concerning the problem. 

Nice concept, but I'm afraid you should brace yourself against being gamed A LOT. Make sure you payout less than what your local wholesale club charges per-unit for a giant pack of candy & chips. Or pay out bitcoin/vouchers/"Rev Bux (TM)".

I think this is a very challenging implementation and a wonderful learning opportunity. It's easy to verify currency and spit out junk. How are you going to verify that you aren't receiving empty wrappers, soda bottles filled with water or maybe a neatly glued-back-together bag of chips filled with poop??

Because we all know your middle school peers are already plotting what I'm writing :)

I think it depends on what you're goal is. if you want to learn about building cool machines, I think there are better projects with lower chances of human excrement getting all up in your circuits. OTOH, if you're serious about reducing junk food in schools, then I would recommend a social engineering approach.

Too many good engineers have already been lost throwing more machines at social issues.

Oh believe me I am the kind of person who would put in a Doritos bag full of rocks. For one, it doesn't start to so anything with the food until the Plexiglas door has closed, making it inaccessible to the user, and with a solenoid/spring keeping the storage bay closed so nobody can reach in and steal "exchanged" snack, like a food airlock. It has a postal scale that I'll post an update on soon, and the device checks the weight and scans the UPC for every product. I don't know how many people would bother to fill a bag of rocks to exactly 3.4 oz (since most foods are packaged by weight, these numbers are within 20 grams in my experience) and then print out a UPC. Now I was thinking that at least at my school they already have teachers monitoring the lunchroom (close to the vending machines), so nobody could really poop in there without getting suspended. I agree, in any sort of setting open to the public it would be a pretty crappy idea, maybe even literally. But I have enough trust, at least in those at my school, to think it will work.

In the app I have on the repo, the admins can set the foods they want to accept, so if something bad does happen, they can block that UPC, category of food, or manufacturer. You can block by ingredient, too, in, say, a peanut-free school. I know that before this could go anywhere one would have to consider things like that, which kind of felt like localizing apps, where it doesn't directly benefit most people, but is a deal breaker for some.

The default return price is 75 percent of the 1st quartile price of ones from a few different websites, so it's not like somebody could have an endless cycle of buying and exchanging Doritos or anything. I'm assuming resale is out of the option, since who knows where those snacks have been?

P.S. If someone is not only taking the time to fill a chip bag with something nasty, but also measure it out within a 30-gram range, they can take their stupid 50 cents; they've earned it.

My mom used to work in market research, and helped me get ideas for ways to stop stuff like what you said. I also looked at those phone recycle things and ATM's for inspiration.

Man I wrote a lot, probably should have made this a project log update!

Also having the option of paying out bitcoin would be kewl, but a. I don't know any (other) middle schoolers who have bitcoin wallets b. The entire system runs without an internet connection in its current state and i dont want to make it need one and c. Bitcoin transactions are harder than cash

Also, does bribery count as a social engineering approach? ;)

Sure it does. Sounds like a good plan. I'd love to see an update.

Would suggest that instead of giving out cash, you give out milk/veggies instead. Less likely to get misused that way and more likely to be actually getting kids healthier. Unfortunately it does add cost due to required refrigeration and regular re-stocking. 

That was my original idea, but I don't think that as many people would want to just replace their Doritos or whatever with carrots. As a middle schooler with a penchant for certain snack foods, it's going to be a pretty hard sell even for cash. Also I was thinking the refund would be like half the retail value, not full price. I've got some ideas for food verification to keep out the riff raff, though.

GREAT!  :D