I use it for craigslist. You can RSS exact searches. I have an RSS feed on my igoogle page, and every time i open up my web browser, i can check craigslist for all the items on CL I am looking for in about 10 seconds.
You just make the exact search you want on Craigslist (be sure to refine it as much as possible. Like my arcade search is something like: Arcade - fire -xbox -playstation -"fisher-price")
Once you got the search putting the results you want, click the rss button on the bottom of the screen. Copy the web page link, not the body, and paste it into your RSS program. Viola, you will now have an updated feed of your search.
I have never used it for ebay, mostly because I get email alerts for that site. Time is usually not so much of the essence, so a daily update is good for me. The few items I am desperately looking for, I get an instant email alert on.