A great tool to add to your website due diligence arsenal, the Wayback Machine is a digital archive of web pages across time. A website time line if you will, the Wayback Machine gives you the ability to see how a website has changed over time and lets you cross reference the website established date claimed by the seller.
For example, you come across a website listing on Flippa that claims to be established 2 or 3 years ago. Running the domain through the Wayback Machine will let you view, and do some limited browsing of, the site when it first appeared. Step back in time and see what sort of content was on the site over time.
The following Wayback Machine results for Flippa.com indicate that the domain existed in a previous life back in 2002. It also gives an indication of the activity on the domain:
When using this due-diligence tool keep in mind that Wayback Machine snapshots only become available 6 to 18 months after they are archived.