Check the demographics. Publishers rely on three methods to get these demographics. First, with inference, a publisher will make the usually fair assumption that the site's content attracts the right audience or is similar to its print demographics. Second, registration allows a publisher to collect much more detailed demographics, but very rarely is an entire site behind a registration wall; so the registration data is only a sample subset of the most engaged readers of the site. Finally, the survey method is the most common. It surveys a sampling of the Web audience. Make sure the publisher's survey is no more than a year old.
Pay more attention to impressions than traffic. You hardly ever buy an ad that appears to every person that visits the site and every page they visit. Instead you typically buy a specific number of impressions, a portion of the site's total traffic. So in the end, total site traffic really doesn't matter to you. What matters is how many times your ad or sponsorship will be seen for the money that you are spending.
Source: Eric Shanfelt, exec VP-e-media, Virgo Publishing