So I guess we shouldn't bring up the time you lent a guitar to a guy for an hour and he covered your Zehphrs in blood?
I mean, it's a nice idea and all but not really original in that many music stores already rent gear. The way you're doing it, while well intentioned, is simply very risky due to the fact that you aren't a business and don't have insurance to cover loss/theft/damage in the event that something unfortunate may eventually happen. You seem very adverse to the idea of profit, but if it were me I'd at the very least get the needed safeguards in place and charge what you need to break even.