With carmax the quote they give you depends on their current supply of that car and the demand for it at the time...on a national level. So just because your local carmax might be out of s197's doesn't mean they'll offer you more. Check their website and search "at any distance" to see how many s197's they have before getting an appraisal...if you time it right you will get a higher quote. Also for a sports car you want to do the trade in the early summer when demand is highest.
I bought a 07 ram new for 16.5k and traded it to carmax 2 years and 30k miles later for 15k...they ended up selling it for 18.5k (I stalked my old truck on their website lol). They didn't have many single cab hemi rams at the time and it was in early winter.