On the issue of people dropping off a bike and never coming back.
I know a couple of repair shops (not bicycle) that post a storage fee, one is $10 a day everyday after they call you to pickup your repaired mower. Once the storage fee exceeds the value of the mower, they sell the mower.
You could always start by just reselling used bikes, it would be easier to set your hours and test the waters. It could also be run by anyone, meaning you personally would not have to be around to make the sale, where you would have to be around to quote a repair...