Where I work some of us regular shop in larger volumes and brings soft drinks etc to work on a common courtesy system. As in we bring it in and expect people to pay pr drink/eat that they take from the fridge. We have however discovered that some help themselves more to stuff then they pay for and we have looked somewhat into commercial dispensing machines but found them rather pricy and looked at each other and decided to look at the BYOD(Build Your Own Device) path.
Ie either building it completely from scratch, or finding a deflunct machine and then revitalising it with an Arduino or etc to control and dispense drinks and assign people credit to buy from, either via swipe card, prox tag or other such. As in people swipe/present and then picks a drink and their credit gets deducted accordingly.
But is that a complete longshot or is it a feasible idea and one that we can get on with at a cost lower then what it would cost to achieve an actual machine out of the box?
The mechanical system is obviously beyond the scope of this Forum. And I guess you need to have a system, or a plan for it, before you can figure out how to use an Arduino to control it.
The payments side could also be complex. I would not consider anything involving real credit/debit cards. You could make some form of "equivalent" with an RFID device. You will have to decide how much security needs to be built in to avoid cheating.
After that the Arduino coding would probably be reasonably straightforward.
The plan was to use RFID chips as we already use these at work for controlling building entry and such. So it would make sense to use the same chips to assign people an account and thus credit signifying how much they could buy before talking to me or my collegue about having their saldo updated.