Google Webmaster Tools gives webmasters access to an enormous amount of data about their web sites.

Google trusts to give the data out based upon the presence of a text file on the server or a meta tag in the web site's index page.

The text files have names like google85605f57d0fe7f5f.html. Each filename unique represents a single account at Google Webmaster Tools.

The meta tags operate in the same manner:
<meta name="verify-v1" content="wyrZZnCK+XCCLodMvqhv/89bsX0824LXw1CE65886Sg=" />
If someone is able to put this meta tag into the index page of your web site or drop this file into the home directory of your web site, they will gain access to all of the data Google Webmaster Tools publishes about your sites.

This is a reminder to delete these files and these tags when you purchase a web site. If you forget to do this, the seller will continue to have access to all Google Webmaster Tools data for the web site.