Running a small business is a 24/7 operation. You don't get no vacation nor sick leave nor any sort of benefits. Everyone has their hand out to you, usually thinking that since you're a businessman you've got lots of money they can squeeze out of you.
A dealership, small pop or carvana does not go into used car auctions to overbid, to sell at a loss. Car Dealerships also operate trade in strategies to grab cars to resell at below market price.
Carvana is losing money. In one year their losses for q3 increased by 200 million. You can apply all the fancy metrics you want about how much profit each car generates, but you still have a company that shouldn't be able to continue business without huge investor backing.
On the other hand, if you are operating at a loss, you will be able to continue doing so indefinitely via continuous cash infusion by investors (which might include the public in case of a public company)... until the cash does run out.