I recently sold my 2003 car with 150k miles to Carmax. I found it to be a decent 1/2 way point between the speed of trade in and the better price of a private party sale. Allegedly I could have sold for about $3k privately, dealer offered $1k trade, and Carmax gave me $2k for it on the spot. The dealer didn't even try to compete. In fact, I drove to Carmax, sold the thing, and he picked me up to buy the new car. They only sell newer cars on their site, but apparently most of their money is made by auctioning off older, and more well-used cars in their own auctions.
If you have the time/inclination, I suggest getting a quote from Carmax (which is good for 7 days), trying to sell it for more than that 7 days. If nobody beats your carmax offer in 7 days, sell it to them and move on.