Not everyone enters the realm of website publishing by building a website, some people choose to purchase already made websites. The goal is to find a website who's owner is not entirely adept at promotion or revenue generation. This way he will likely be willing to sell the website for a small amount, because he doesn't see much worth it. You on the other hand are an expert at promotion and revenue generation and so you'll get the website for much less than it is worth to you.
Unfortunately there are a lot of website lemons out there, the purpose of this article is to provide you with the tools you need to find that diamond in the rough.
There are two tools you absolutely need to be familiar with if you plan on purchasing a website, the Google toolbar, and Alexa.
The Google Toolbar, when installed with advanced features, will give you quick access to the PageRank of any site you visit. If you know your search engine optimization, and if you know what you can accomplish with PageRank of varying amounts, then knowing the PageRank of the site you plan to purchase will give you an idea of how much the site is worth to you.
Often times the owner of an ecommerce site might want to purchase a content site for promotional purposes. Knowing the PageRank, and specifically the Overall PageRank Power, of the site you plan to purchase will let you know exactly what kind of benefit you can expect to see if you cross promotion your current site and the purchased one.
Be wary about PageRank though, it can be manipulated. You need to check the page's actual backwards links (also using the toolbar) and see if the links are from good sources, like Yahoo or DMOZ, or if they're simply from the owner's other sites. If the later then that PageRank will likely evaporate as soon as the site is sold.
The second tool is Alexa. Alexa collects traffic statistics for millions of websites by monitoring the surfing habits of its millions of users. These statistics are more or less accurate for high end sites, those with ranks of 100k or less, but can still be manipulated somewhat by the site owner. Webmaster sites or those that cater to webmasters typically have a higher rating than normal because on average webmasters are more likely to have Alexa installed.
Alexa will let you verify the traffic of any potential purchase so that you will be able to discover if the site owner is embellishing the statistics he reports to you. By knowing the traffic of a potential purchase you will be able to estimate how much direct revenue you will be able to pull off that site.