I'll answer that: me.
I'd classify myself as a "car guy". I do my own wrenching, worked as a mechanic for a while, have the big ol' set of tools (though in a Craftsman box, not a Snap-on), and have a special love/hate of VW products....like I think anyone who works on them does. I'd normally never even consider buying a car sight-unseen. Until last year.
We live in the NorthEast United States. My wife, with a short commute and Level 2 chargers at work for free, decided that it was time for her 2016 VW Golf to go. The perfect car presented itself after a short search: 2017 VW eGolf. Produced 2014-2019(?), it was all of the Golf goodness, but battery powered. Has a range of ~100 miles. It was, effectively, the test platform for what became the now-produced ID.4.
Here's the problem: VW never sold them in this part of the country. Something like 90% of them were sold in California, so they're literally not on the used car lots in this part of the country. The closest one was something like 1000 mile drive from here. Even if we went out there to get it, getting it home would be an adventure unless we shipped it.
Enter Carvana. They had a half-dozen available; the prices were reasonable. We picked the colour and options we liked, and for 750 bucks delivery fee and a week later, the car, which was originally titled in California, showed up in front of our house on a little rollback. The deal was this: you had 7 days and 300 miles to test drive the car. If you didn't want in that period, you called them up and they'd come get it. You'd be out 500 bucks for shipping fees if you did that, but that's it. For a car that we wouldn't have access to otherwise, 500 bucks was a reasonable chance to take.
Turned out that the car was even nicer than the pictures let on. Granted, this isn't a tuner kind of car, so if we were looking at WRX EVO's or something, I'd have been way more wary of it having the shit beaten out of it. But, those are available here though conventional means. The eGolf wasn't.
This, I think, was Carvana's key bit of power: access. For run-of-the-mill cars, who cared. For the harder to find cars, but not the exotics that have their own following, they made it very easy to have access. Cars like the eGolf, Like the Fiat 500's Abarths, like Audi TT's, like the WRX's. Not rare cars, but not common on the used market in the less populated areas of the country. They found a good niche that was underserved in the car world.....and then screwed themselves.