Its a trend of webmasters to buy old or expired domains. This is for getting more SEO benifits quickly.
But you should be very careful while buying expired domain names. The reason is that, the previous history of that domain may sometimes trouble you.
So double check the domain history using way back system to verify that the site was used for good purpose.
The second thing to check is the listing of that domain name in search engines.
If you are buying a PR website, then make sure that the PR is not fake.
If you consider above factors, you will be benifited from second hand websites.