Agree with used. I've gotten both my cars from Carvana and it has worked out quite well; saving over 10K per car by going for a 2 year old car each time.
They're under no legal obligation to cancel the transaction if your post-purchase inspection turns up something. In almost every state, used cars are sold AS-IS. It's entirely a 'good will' policy and from this video it sounds like they won't give you money back, but instead allow you to swap with another vehicle in their inventory.