The sports car market drops hard in the winter, since most of the country has frozen roads on their mind. So, it's the best time to buy. They are upwards of 20% cheaper from dealers because no one wants them sitting on their lots, and on their books for 4+ months.
Getting a good price from Carmax is rare though, since they can, and will ship to their locations all over the country. If it didn't sell over the summer/fall and wasn't getting much action on the website (dunno if they track this), they could have brought it down, fairly close to dealer auction price (within 10% - 15%), just so they didn't have to put it on a truck to Manheim (largest auction co. in US).
When I had access to Manheim's MMR pricing tools (black book), the cars I looked at on Carmax, had upwards of a 30% markup over the dealer auction prices. Therefor, I ended up rarely checking Carmax because they were always bad, so I am sure I missed a decent price or two.
As an aside:
When I was looking for my car, 6 years ago, the only "dealer" that I found that was consistently close to dealer auction price (at least for the make/model I wanted) was an Ebay seller called Texas Direct Auto (usually under 15% markup). Their whole business model was thin margins on high volume, directly from auction. So they paid all their employees hourly and kept a minimal (under) staff. For the vehicles, all they do it detail them, snap some pictures, and move it out the door. I didn't buy from them, but I would use them as a starting point to check prices, if I was looking today.
I even made a short spreadsheet when I found the exact car that they bought in the Manheim comps list (they auction so many cars, it's usually too difficult to find the exact auction):
And here are some screens of what the dealer pricing tool looks like at Manheim (from 2014), if anyone is interested (I dont have access anymore, but it was fun to play with):