They are selling Drupal modules for money - what is permitted by Drupal's GNU GENERAL PUBLIC LICENSE Version 2 - but also they are restricting alterations and copying. As I know all Drupal modules inherit the GPL licence from Drupal.
I think they basically break this with their software licence agreement what you have to accept while purchasing a module from them.
Here are the 2 most important parts of their licence agreement:
"5.2 No Alterations
You may not modify, adapt, translate or create derivative works based upon the Software."
You may not rent, lease, sublicense, assign or transfer your rights in the Software, or authorise all or any portion of the Software to be copied onto the computer of another user or another legal entity, except in expressly authorised cases."
Is this legal?
What if someone buys a module from sellxed and uploads it to drupal.org as a contrib module? ;-)