Well, CarMax is a dealer. They have the same business model as any other car dealer - buy low and sell high. I don't know why you would rely on them to give you a better offer than any other car dealership. Expect to get $4-5k less than you paid for it. The biggest depreciation hits come on a car during the first year and then again after the third year.
The '07's are being discounted a bit more now than they were when the model year was new. My sister just bought an ES for $1,500 less than they were selling for last fall when she was first given a quote on an identical model - from the same dealer. And since the '08 ES will be here in August, I expect the '07 will become even cheaper.
