+1 to what sicboater and BeBerlin said about RSS feeds.
They are just brilliant for keeping track of new listings without even having to pay attention.
You don't even have to install any software to be able to use RSS, many e-mail programs has some support for RSS, or you can do what I do and use Google Reader (http://www.google.com/reader) to keep track of as many RSS feeds as you would like.
Why not try it out? Add the classifieds RSS, add a few of your favorite newspapers RSS feeds (almost every newspaper, magazine and sportssite on the web has a RSS feed, look for a orange symbole thats either says RSS or XML). Google Reader is free to use, all you need is a free Google account, and you can use Google Reader from any computer in the world, as long as it is connected to the internett, without installing a thing.
Arlendius