"Similar to when General Motors launched the Saturn line of cars with flat pricing"
And we all know now how well THAT worked out. Haggling, whether successful or not, gives the impression to the customer that they are in control and are 'saving' money.
In the early days people loved the flat pricing and no haggling, and their Saturns. It wasn't until 96-97 when Saturn became just another Lego set GM marque, complete with haggling, that it all went down hill.
My guess would be they will haggle if the buyer wants to haggle. They aren't going to give up a sale. But, I've never shopped CarMax so I don't really know.
I had a roommate years ago that worked as a salesman at CarMax. He had about the authority to haggle as the kid taking your order at McDonalds. I think the volume is so high that if you don't want it at the marked price, someone tomorrow will. Of course, that was years ago at one lot. YMMV.
And we all know now how well THAT worked out. Haggling, whether successful or not, gives the impression to the customer that they are in control and are 'saving' money.