To piggyback on what smedleyb is saying you have to do some market research on your own, not just consult KBB and Edmunds. KBB is little more than a benchmark. I have sold five cars on CL and have had a lot of success on there with this four step approach:
1. Search CL and Autotrader...figure out what private sellers are charging and what dealers are charging. You are a private seller, but want to be aware of the dealers as well. Choose a price...how fast do you want to sell it. Obviously, cheaper sells faster. The more expensive the used car the longer it takes to sell...even if it is priced in line with value. Most people looking to spend $15-20K on a car will go to a dealer. It's going to take some time for $15K car to sell on CL.
2. Clean your car up and stage it well. Take nice photos like MMM suggest. Use this link for instructions so you get nice big photos in your ads
http://smallnotebook.org/2010/04/14/craigslist-photo-tip-how-to-show-big-clear-photos-so-your-stuff-sells/3. I like to use a three part approach to the narrative. First, list the specifics to your car in a couple of short sentences (mileage, VIN, whatever you want, but I put enough in there people aren't going to call or email me). Second, go to a car web-site and copy and paste a professional description of your car into the body of the ad. Choose one that sounds a little sexy, not just the first one you find. Third, close with your contact info and let them know to have financing arranged BEFORE they call you. I like to include how the deal will be done in that part as well. For example, "we'll meet at any Chase Bank branch to complete the sell and sign paperwork" (BTW, don't let them come to your house...you don't want them knowing where you live). I do whatever I can to make the people feel safe and secure with the purchase...I think closing in a bank helps and I have done this on the cars over $10K.
4. Post and re-evaluate. If you haven't had any movement in a week repost the ad. I like to repost every other day. After a month drop the price. After two months, make a new ad from scratch and consider another price drop.
At the end of the day, most of it comes down to price, but a nice ad goes a long long way. Don't underestimate it's value.