Yes, it is perfectly legal to sell any software which is under the GPL License, as Drupal is. The author of these module, Leighton Whiting, felt that it would be better to sell these modules so that better support and faster upgrades would be possible for the serious users of drupal who need high quality modules for their sites and businesses. Leighton Whiting is the maintainer for many modules which he has released on Drupal.org for free, among them are Click2Sell Suite and Role Contracts.

