As recently as last year, it wasn't clear that RelayRides' business made financial sense. The company, which lets strangers rent one another's cars, was growing -- acquiring rivals like Wheelz and expanding into new markets. It was charging its users a healthy vigorish; car owners paid the company 25% of every rental fee, while renters kicked in an additional 15%. (Most digital marketplaces only take a total of 10-20% of every transaction.) And yet, the company was still losing money on every transaction. The culprit? Insurance.