You don't really have to sell it yourself. Many of the larger used cars dealers realized a few years back they could get better deals buying late model cars form the general public, and then webuyanycar.com joined the fray. Assuming yours is a clean 2016 cla with about 15k miles, you could walk in tomorrow and get a check for $22,315.
Selling the same car yourself, you should expect to get at least 5,000 more. Up to you if it's worth 5k to sell on your own. If you do, cars.com , autotrader, eBay are all popular, Craigslist sells a lot of cars, but not sure at this price point?