The well-established model for a lot of online retailers is, you go and look at the thing in a normal shop, but stop short of buying it there. Instead, you come home, browse the internet and choose on price. Great for guitars, laptops, tellys etc and most things new, helped by DSR meaning you can send it back within 14 days. Unless the cars they source are absolutely fault-free and they somehow build a reputation for great cars, I can't see it ever working for an online secondhand car proposition. My experience is the best cars go new>used but stay within a dealer group eg Inchcape, Arnold Clark etc but even then, their processes aren't robust enough to filter out bad cars or afford to go the extra mile to resolve an issue, they dig their heels in and treat a car just like any other dealer treats a car comeback. The reviews read generally good but there are always some horror stories.
So, they need to "break the ceiling" for customer care; but do it at the cheapest price, cheaper than everyone else. From a starting point of nothing. Can this be done simply by relying on economy of scale? With someone else's money? Hmmmmmm I'm oot!