Not a broker agreement, but a template for Domain Name Purchase Agreement. You can modify this document.
I've got this template from someone here or other multiforums, I forgot his/her name. I'm using this agreement since 2007 if the buyer requires a similar agreement.
Here's the link, and if you modify the document please share with me too.
http://www.dncon.com/groups/114556
Cheers,
EM @ KING.NET