I sold my last car to CarMax and bought my SHO from CarMax, so here is my story:
CarMax does not use Manheim, KBB or NADA guides, they use the pricing of vehicles they sell directly from their lots. This means that in your area, if the CarMax is selling a car that sells quickly and for top dollar, you will get top dollar when you sell it to them.
Sold my 2012 Acura RDX to them, this is how it went:
I was looking for a best case scenario of $29,000. I went to the Acura dealer I bought it from, had all the service done there, and they offered me $27,500. I went to the Cadillac dealer, they drove it around, offered me $27,000. Frustrated, and thinking CarMax would offer me even less, they came back with $30,000. I Was incredibly surprised, but the RDX sold very well out here in CO on their lots.
I went 4 months without my own car, and carpooled with my wife while I searched for my new car, decided on the SHO, looked all over town and they were all right around $28,000 - $29,000 at that time. I went online and decided to check CarMax's prices, and the SHO's that were local were all in line with that pricing, but I noticed that in Southern California, they were selling for around $2,000 less. This is because they price the cars for their market. With their transfer service, I bought my SHO for $25,997 and paid a $399 transfer fee to get it out here.
SoCal has no premium on AWD models, as it is SoCal and has no need. Out here in CO, they sell for a premium.
A few other things with CarMax: if your car has over 100K miles, or is much older, they will give you dirt for them. That is due to the fact that they won't sell it on their lot, so they give you auction pricing. If your car doesn't sell well on their lots in your area, they will give you dirt.
I have bought over 20 cars in my life, and CarMax was by far the best experience. No, they won't haggle with you, and the price they give you when you sell your car is set in stone based on what the market value for them off their lots are.
Not sure what the SHO would go for, as Darkside said, not too many people know what the SHO is or are looking for it. CarMax is a 7 day pricing offer, so you could check there first, then compare. A lot of dealers will work with you to get over what CarMax offered, so you can use it as a tool.