Carmax will offer you exactly 'wholesale' or 'black book' value. This is where most dealers will start when negotiating your car as a trade. You only get more out of the dealer when they start negotioting away their margin on the car you're buying. They're never really giving you more for your car.
Of course, now that you've bought the other car, you lose the tax advantage of the trade so the Carmax offer is really worth that much less. The last car I had appraised at Carmax was my '02 e46, they offered $7k and I sold it through Autotrader in 2 weeks for $11k.
My advice; Price it $500-700 less than the most comparable cars. Take a bunch of really nice pics. MOST IMPORTANT; treat every potential buyer seriously. You may waste a bunch of time returning calls and answering emails but you never know which of those will turn out to be for real. IOW, Autotrader is easy money as long as you're willing to put in more effort than all the other sellers. Good luck