Most of us use Craigslist to shop for drums and cymbals and we all have probably been to late when finding a good deal. There is a software called Craigslist Notifier that I am considering purchasing. It seems safe and I have read some good reviews. Although anything you download is at your own risk.
Below is a link to the site selling this. There is a demonstration video you can watch and a free demo download but it is limited compared to the full version. I think it cost $10.00. The cool thing is that it notifies you within 5 minutes that something has been posted on CL based on the criteria you have set up. Check it out and watch the video.
