Finding Expired Domains
You might wonder why, exactly, anyone would be interested in investing in a used domain. Well, an obvious reason is that, perhaps, they're just really fond of that particular domain and want that name associated with their business. While this can be true, it's much more likely that an owner might invest in a used domain to capitalize on the previous domain owner's success and work.
Quite a bit of time, money, and effort goes into making a domain popular. Page rank, directory listings, back links, and all other sorts of things are required to make sure that a domain appears frequently and near the top of a list of search results. In other words, it's hard work to make sure a domain gets to the level it needs to be at to properly promote a business. For an owner who has the physical equity of knowing how to do these things, the process can be time consuming and swallow up time that could be properly used to promote other aspects of the business. For the owner who does not have the physical equity to do so, pushing a domain through the ranks will require hiring an expensive professional, so right there quite a bit of money is spent. Additionally, you lose the time it takes for the domain to be pushed towards the top, and if time is money, well, that's even more money out the window.
Basically, you might invest in a used domain that is expired so that you don't have to do all of this hard work yourself and you don't have to pay someone to do it for you. Not only is it possible, it is common for domains that already have quite a significant amount of time and effort invested in them to become expired for a variety of reasons. Now it becomes possible for interested parties to acquire the domain, very often at a cost significantly cheaper than it would be to invest all of the time and money necessary into pushing a new domain into a similar position. Essentially, buying a used domain puts an owner in a position where they can immediately capitalize on the domains success and popularity to recruit traffic for their own purposes. What this means for the owner is that they have essentially gotten a ton of inexpensive, effective advertising that is constantly promoting their business. In other words, for a little more money up front an owner can save an immense amount of money later.