Carvana is stupid easy and convenient to buy from, but I'm not convinced they really do their homework in terms of checking the cars they're selling. If I recall correctly they give you a week to return the car, and I'd definitely get an inspection of the car done in that time since it isn't an option pre-purchase. After those seven days pass it's your problem, not Carvana's, so caveat emptor and all that.
The one caveat is that if you've traded in a car and end up returning the one you bought,
you can't get your trade-in back. They treat it as a separate transaction so in reality you're not trading it in, you're selling it to them.