CarMax you say? They buy cars from people, whether those people buy one of their cars or not. And they only pay trade-in value. So at $16K I'm guessing they're making a good profit. I'd ask for a lower price IMO, though they said it'* non-negotiable.
I'm not sure if CarMax'* policy is to not negotiate prices. I know some places that do not, but their prices are LOW (I bought a car from a place that does not negotiate, but I got it for $3000+ below retail). $16K is pretty much TOP dollar for that SLE. You could probably find an SSEi of that year with the same miles for the close to the same price.
However, 18,000 miles is really low. If you're not really interested in an SSEi it could be a great buy.
(I still think they're asking too much