We have a trust-based beverage PoS at our hackerspace. The bottle crates are just standing there and everyone is trusted to pay into the cash desk next to the crates when they take bottles out. Our weekly losses (i.e. bottles and money not accounted for) average ~40€, with weekly revenue around 200-300€. The losses are factored into the prices. Prices are comparable to or slightly below convenience store levels.
Is there a mechanism for IoUs / credit lines for members?
I've seen trust-based mate shops at hackerspaces around the world, and I've been told that the ones that have a credit system (even one that's not limited, checked, and that's not even authenticated) end up actually not having any significant losses. Like, anything from a piece of paper with people noting down unpaid mate purchases to things like android tablets with a webapp letting you note that you put in a bill of N euros and taken out a bottle of mate.
Friction from being able to pay right away seems to lead to "I'll pay extra later" thinking, which makes people either forget or miscalculate how much they actually owe.
Yes, each member account has a balance without any restrictions in either direction, and it mostly works fine. At times, the negative balances pile up, in which case I write a mail to the mailing list asking people to pay their dues. Usually, the negative balances approximately cancel out the positive balances, both at something like 1000 euros.
Also, when your balance is negative, after every purchase there is a big red notification reminding you of that.
