I have had good luck finding expired domains that are also in Yahoo. I do make sure they are on subject though.
Another method not mentioned here, is finding non-domain name accounts in Yahoo.
For example, If you sell Widgets, you might find a Widget site with a URL like this:
If this URL doesn't work when you click on it, you might contact Earthlink and see if you can sign up for an account with them using the name WIDGETS. Sure, it might cost you $20 a month, but it's cheaper than the $300 a year Yahoo charges.
I have gotten a couple listings in Yahoo like this. However, I only do it if the URL has lots of other links throughout the web.
There are several utilities on the web where you can check to see where the URL is linked. I use the Bruce Clay tools. Just search Google for "Link Popularity" and you will find several good tools.
One of them I picked up had nearly 1,000 links around the web, and was ranked #2 on Yahoo for my most important keyword. And, since I now owned the account the original guy used to sign up for Yahoo, I was able to email Yahoo and change the listing description. You would now own the email address the original guy used to sign up for the Yahoo listing (Widgets@earthlink.net). Yahoo asks for the original email when you try to change the listing.
hope this helps.