Get a 3rd party inspection on your car before your 5 day return policy ends. You absolutely must do this before your 30-day warranty ends.
I bought an excellent-condition "certified used" truck from CarMax. Sales process was great. In-and-out in under 1 hour.
Literally on my way home, I noticed some issues with the steering. Complained and took the truck in to the CarMax service center. Turns out, their mechanic found four issues with the vehicle --all which should have been found in their 125 point inspection (wheel rim was not circular, alignment off, parking brake broken, steering wheel not centered, rear differential making loud noise).
CarMax took care of it and fixed everything, but my truck was in the service center for over 21 calendar days.
Overall, I would recommend CarMax, but do yourself a favor and get your vehicle independently checked after purchase.