Firstly I'd email the person who put the brushes up. They're listed on the site as sarthony@gmail.com Take it gently - they've only sold 6 copies - and the easiest way this is resolved is that they apologise, take them down and maybe send you $20 via paypal

If they dispute your ownership, contact the people behind the site and point them to your thread where you create the brushes. The joy of forum posts is that they're dated At that point the user should get the message and have them taken down.

On the plus side - sarthony has shown that people are willing to pay for your brushes!