If you buy keyword specific domains, you're really buying the type in traffic. I use the URL builder and redirect through that URL so you can see how much traffic your getting from the keyword domain. There seems to be no rythme or reason to what keyword domains deliver traffic and what don't. By tracking traffic with the Google URL builder you get a feel for what names are giving you traffic and which are not. ie. the plural, the singular, two words, three words, the possessive, etc.
It is very possible that the owner has a recurring payment in place, or that the owner renews after it expires. Because GoDaddy gives a redemption period like all registrar, many domain name investors will actually allow their domains to go through auction to see what the “market value” is of a domain name and THEN renew it. It sucks if you’re the buyer thinking you won the auction. It happens often, unfortunately.
For many reasons, these domain names expire and become available to the market. Perhaps the domain renewal fees are unpaid. The owner no longer needs the domain or business came to a halt. With the large pool of domain names that become invalid daily, investors who are looking to cash in on the traffic that they generate have to do extra work to find the most profitable every day. The trick is to find expired domains.