All good things must come to an end, and this is also applicable to high-ranking websites. At times, there is simply no point maintaining them, simply because there is nothing left to explore. But just because a site is about to be shut down doesn't mean it should go to waste.
One way to cash in on a closed site is to sell the domain name.
Of course, before you sell your domain name, you'll have to notify your registrar or web hosting company first just to make sure that you are not violating terms in your contract.
The next question you need to ask yourself is how do you know the value of a domain name? A good place to start would be checking with an appraisal site. Basically, these are sites that assess the value of domain names. Here's a list of the most reliable appraisal sites:
Estibot
This is one of the most trusted appraisal sites in the industry. It claims to conduct 1.8 million appraisals each day.
Estibot's results page contains various domain details (domain, SLD, and keyword) right down to its estimated value. One of the most useful pieces of information on the results page is the availability of the domain name under several TLDs. Other information the results page displays includes the average search result, and traffic rank.
DOMAININDEX
Another free public service appraisal site, DOMAININDEX also offers one of the most detailed results page. Domainindex is one of the few sites that tracks the development of the domain's price level, indicated through a line chart.
For those that want more comprehensive results, they can subscribe to any of their appraisal packages. Their starter package, which allows users seven appraisals a day, is absolute free.
Valuate
Aside from the usual details (e.g. value, ranking, searches), this site also shows the Alexa ranking. This is a ranking system set up by alexa.com that measures the number of visits of each site. In valuate.com, the Alexa ranking is displayed along the Traffic column.
These are just three of the leading appraisal sites. If you want a more comprehensive analysis of your domain's value, a good idea is to use several sites and base your decision on the average.
The next step is to find a buyer that would be willing to pay for the price you set. There are several sites where you can sell your domain name.