This means:
Anybody that purchases it can then go ahead and give it out for free or even use it in their own commercial project. The main Wordpress site could also give it out for free as part of their services.
I steer clear of all open source projects (aside from some libraries) when it comes to commercial apps.
Those that don't just don't mention it, like WooThemes. They're one of those millions-a-year businesses. You won't find GPL or any license terms mentioned on their site or during checkout unless seek out their "Terms and Conditions" page yourself -- a link they hide as much as possible -- black text that doesn't look like a link under the logo in the footer, apart from even the other footer links.
In reality, the licensing issue isn't a big deal. There are over 50 million active WordPress installs -- it's a huge market. If you provide an awesome plugin, and you support it, people will pay you for it, even if they could download it off some warez site and you'd have no recourse to go after them.
Part of this license outlines requirements for derivative works, such as plugins or themes. Derivatives of WordPress code inherit the GPL license. Drupal, which has the same GPL license as WordPress, has an excellent page on licensing as it applies to themes and modules (their word for plugins).
There is some legal grey area regarding what is considered a derivative work, but we feel strongly that plugins and themes are derivative work and thus inherit the GPL license. If you disagree, you might want to consider a non-GPL platform such as Serendipity (BSD license) or Habari (Apache license) instead.