That's unbelievably good for a CarMax offer...they typically give bottom tier (fair) or less for their offers. What's good is that they offer is the same whether you buy a car or not. That's pretty nice.
Having just finished looking for a truck, I can say you're definitely better off going elsewhere for the new truck...CarMax's current prices on Rangers are well above anything you'll find at a Ford dealer. Trucks are a dog right now and the deals are great. When I was shopping, there were used Rangers going for more at CarMax than what the Ford dealer was asking for new ones that were similar. The deals are even better on the Mazdas.