You'd probably get the most value selling it yourself through ebay or kijiji, but you have to be ready in dealing with the headache and scams. Also, people don't like buying laptop from seller who is not a powerseller (with many positive feedbacks as a seller).
Instead, you can try your local authorized Apple dealers (not Apple stores, but still sell Apple machines). Most of them sell (and therefore buy) used computers. You are not likely to get the best price, but the beauty is sparing you from any hassle. Also, you will be able to save on sale tax when you buy your new macbook at the same time - only tax on the difference. So you do get a little more than you think.