I've rarely had problems selling items on CL. To avoid too much hassle from flakers, I will usually tell them a fairly specific time when they can show up, and make sure it's a time when I'm planning to be around the house anyway. If they flake, I move onto the next person immediately. It can feel a little harsh, but it definitely prevents situations where you feel like you're getting totally dicked around for $5 or $10. I also think there's a certain art to deciding on the correct price point for minimizing flakes. In my experience, when I have posted at a rock-bottom price, I'm more likely to attract flakers. Silly as it may sound, asking for a more mid-range price seems to attract more "serious" buyers. One time I was trying to sell a mattress for $25 and I had a string of really ridiculous flakes (i.e., not just no-showing, but calling to arrange different times and then no-showing AGAIN, and AGAIN, and leaving really long phone messages to explain why. BLERGH). After the last flake took the mattress, she had a huge fight with her roommate on the way home and decided she was moving out, and the two of them showed up at my place 20 minutes later with the girl who bought the mattress trying to return it while the roommate insisted she still wanted it! This was after they had also spent an hour or so dithering around about how to transport the mattress. To prevent a total bitchfight from going on on my doorstep (and because the roommate was totally rubbing me the wrong way), I took the mattress back (sigh). I reposted it for $60, and 24 hours later someone showed up with money, a car, rope, and a squadron of friends to load the mattress up. That was the most dramatic example, but I've had the same experience with other items too--I initially post for a price that's basically "just take this off my hands", get a bunch of interest from flakers, then repost at a higher price and poof! item is gone in 24 hours.
Finally, I find that the timing of a posting makes a big difference to its success. I usually try to post things on Thursday evening or Friday morning: more people are looking to buy on the weekends. In the past, I've posted stuff on Sunday night (after a weekend of organization and decluttering!) and gotten zero bites. Post the next week on Thurs/Friday and suddenly I have people lining up. If you have the time to wait on selling something, you may also want to pay attention to whether or not it's a good time of year to sell something. For example, when I lived in college towns, May/June were terrible times to sell anything because all the students were leaving town for the summer and desperate to ditch as many of their possessions as possible. But come September, the situation was a total reversal and you could (up to a point) name your price.
