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.
Question added to topic SEO | Search Engine Optimization • September 23, 2007
How can I determine the search engine exposure and traffic of a website prior to purchasing?
I am considering purchasing domain names in other countries (example: vacations.com.au). I would like to know ahead of time, before spending the thousands on an international trademark, what kind traffic I can expect from those other countries. This would give me an edge on US competition I currently have. Thanks.