To keep an eye on craigslist for something that you've been searching for for weeks, you can simplify the process by using Mail.app and a craigslist RSS subscription. Simply add an RSS feed to Mail from whatever craigslist category you want in whatever area you are in. After doing this, go to Mail » Preferences » Rules. Make a new Rule and call it whatever you want. Select a new item from the feed, and go to View » Message » Raw Source. Now you can edit your rules to your heart's content. Set the conditions for your rule.
For example, I'm looking for some Olympic Weights on the cheap, so I've got this:
If all of the following conditions are met:
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http://hints.macworld.com/article.php?story=20080111190350828