If you do a lot of business on craigslist.org, it can be quite useful to monitor site activities related to your item(s) of interest. This can be done easily.
The first step is to do a search on Craigslist for the thing in question. On the search results page, notice the box in the address bar that says RSS. Normally when you click this, Safari shows you the site's RSS feed(s). However, (at least on my 10.6.4 iMac), this feed doesn't work; an error message is given. I used Google Reader to get around this error. Before moving on, copy the URL of the feed.
Now go to Google Reader. If you don't have a reader account, you'll have to create one. From any Reader page, click the 'Add a Subscription' button. Paste in the URL of the feed and click Add.
The feed you subscribed to appears in the list at the bottom left. As you hover over its name, a little triangle button appears to the right. Click this and choose New Folder... Type 'Craigslist Alerts' or whatever you want to name it. Now your feed appears in the source list under the folder you just made.
If there are any other Craigslist searches you want to be alerted of, repeat the previous process, but instead of making a new folder, click on the name of the one you created already.
Once you've subscribed to the searches you want, click their folder in the source list. Notice Safari's RSS button. Click the button, bookmark the page, and you'll see it in your bookmarks bar. Whenever new postings are made that match your search, the number of new postings will appear in parentheses after the name of the item in your bookmarks bar. This has been a great help for me, especially when I'm looking to buy something.
[crarko adds: I haven't tested this one.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20101123111209367