Domains that have been registered and never dropped are called "Aged Domains".
These domains normaly sell for more than a new one because they have been around for a while. They're usually out of the Google Sandbox and for those who are looking for aged domains, it can help them manage a living online, having a history, or raise credibility in their niche markets simply because if the domain has been around for years, it appears that they have as well.
Aged domains can also be found on forums like DNForum.com and simply by typing in the keywords "Aged Domains" into the search bar you can easily locate domain auctions that include these older domain names.
I have purchased dozens of domain names for $40 or less that were anywhere from 5 - 10 years old. Just based on the age alone I was able to flip these domain names for over 5x what I paid.
For instance, one domain name I purchased was never used, meaning it had never featured a website on it, it just sat parked in the users account for over six years.
I purchased the domain for only $30.00 and because of its age, I was able to flip it for $379.00.
That's quite a boost in profit from a domain I paid so little for.
While there is no exact science how to flip domains here are a few things to keep in mind:
1) Development Potential
When you analyze the availability of domains in your list, consider what each domain name could represent and be used for when creating a website presence.
An example of which is whether the domain name is one that could represent a product title, or better served as a personal portfolio, a social community, a directory or perhaps a forum.
While the purpose of the domain name will unlikely match your ideas when it is sold, by thinking of a clear purpose for each domain name will not only help you make sound choices during the selection process, but can also be included in a domain auction, as a way of passing on the ideas to prospective buyers who are considering purchase.
2) Length
It can not be said enough, most of the domain names you purchase should be relatively short, basically consisting of two words.
3) Trademark Issues
Avoid registering any domain names that could infringe upon the trademark of existing companies, whether or not you believe that the company will take action or not should not be considered.
The last thing you want is to purchase a domain name that is unable to be sold due to buyers being cautious or concerned of building a website on a domain that ends up being seized by a company wishing to protect their identity.
4) Relevant / Popular Keywords
Does the domain name contain popular keywords that are used by those seeking out more information in search engines? If so, your domain name just increased its value instantly.
One of the easiest ways to determine whether a keyword is a common one is by using the free service available at www.SEOBook.com or www.Overture.com
5) Existing Traffic
If you are purchasing aged or recently expired domains, you will want to determine whether there is existing traffic to the website or not, thus increasing it's value immensely.
Organic, natural traffic sent directly from search engines is the best kind, however back links from other websites are also very important to potential buyers.
An easy way to determine the number of backlinks as well as page rank and other important information is by visiting www.CheckPageRank.net where you can enter in domain names and retrieve useful data relating to the name itself.
When it's time to register your domain names, you can use any registrar that you wish.
Personally, I use www.TLDwebshop.com , a favorite among the domain buyers and sellers. Regardless of the registrar you choose, you will want to make sure that you park them on service sites such as www.Afternic.com or www.Sedo.com so that you are able to generate revenue while you are preparing to sell the domain itself.
6) Spelling
Is the domain name easy to remember? If your customer would purchase the domain and build a business with this name, would he be able to easily brand it?
For example, domain names with double letters such as www.cashhour.com may often be mistaken for www.cashour.com, just the same, domains with odd spellings, hyphens or numbers would have to be clearly spelled out, or explained when someone is attempting to promote their website through word of mouth, rather than in print.
Consider this when registering domain names, and make sure that the names you choose will not be mistaken or misspelled by potential customers of yours, as well as the person who purchases it from you at the last end, otherwise your client will experience a significant loss in perceived value.
When choosing your domain names, there will be many factors that come into play.
The type of audience you are offering to, the auction sites you are featuring them on, the price range you are expecting and so on.
There is no 'one way' to do this, and you will learn to become a better domain evaluator (and purchaser) from hands on experience. Using the guidelines above however, will help you maximize your efforts and minimize your costs (and losses).