Check out Alexa at www.alexa.com and click the Traffic Rankings button on the tool bar at the top of the page. Then type in the domain name of the site that you're planning to buy. Alexa will show you how many users visit the site each day and how the site compares to other sites in the rankings.
How can I determine the search engine exposure and traffic of a website prior to purchasing?
Jon Rognerud is a recognized authority on SEO, who has spent more than 20 years creating and managing web and marketing projects from small to large companies, including positions at online giant Yahoo!. He is the founder of Chaosmap.com, a leading search marketing company in Los Angeles, CA. He plans, builds and delivers profit-making SEO, PPC and Social Media training, consulting as well as breakthrough speaking seminars. He also blogs on his website, http://www.jonrognerud.com