I have to agree with most of the posts here .. you have to be patient and persistent. If something posts up, you have to jump on it right away.
The one thing that I dont find mentioned here though is .. everything is negotiable! It obviously depends on the seller, but I am surprized at how many people think that the price is the price. Whenever I post things up for sale, I assume that there is haggle room in the price. So, don't be scared to make an offer to someone on something that has sat on CL for a few weeks.