CarMax generally does a very good job. They are generally pretty picky about what they will put on their lots.
Prices there are usually a little higher, but not always. Each outlet gets to price the cars how they want. As always, in order to recognize a "deal" you have to know the market well.
Or, you can just not sweat it, go to CarMax, have a pleasant experience, get a good car, and get on with your life. That's their appeal, and they're good at it.
That said, imo used trucks are somewhat of a crapshoot because, unless they are near-new, it seems like most are run hard and put away wet, have a zillion miles, or are modded. I wouldn't want any of that. Me, I'd rather have a new truck and a used trailer than the other way around, but I realize sometimes the resources just aren't there.