CarMax usually gives, at most, the KBB trade-in value. I've looked into selling two cars to them. On one, they would give the trade-in value, on the other, less. Still, it is an easy and fast way to sell. Plus, they will give your car the once-over and give you a quote for the sale and then you can decide.
I also like AutoTrader but don't believe it is the best resource. I prefer
Cars.com. You may also want to check out
CarsDirect. For selling, however, I'd start with free listings, such as Craigslist. It won't hurt anything to try and you don't have to pay listing fees.