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.
Michael Frazier is an SEO expert and Entrepreneur.com's director of ad operations. He has more than four years of SEO and online marketing experience.