I think I've made an expensive error on a car donation

So I donated a vehicle to my local HS's training program and it is a legit place to donate.

 

I got the 1098-C -- 5A is checked and box 7 is not checked.

 

Back when I donated it, I did various searches to determine its average value given its condition: about $14,000.

 

I didn't realize until doing my taxes that I was also going to need an appraisal.  I do have a CarMax offer -- would that suffice for an appraisal?  

 

If CarMax isn't an appraiser, should I call the school and see if they have sold it yet?  The students are not only learning how to repair cars, they also learn how to sell them.  I would imagine given the used car market, they were able to sell at the CarMax offer.

 

Or am I doomed to only taking the $500 deduction?