(Written against GPLv2.)
It is always said that GPL does not prohibit selling the software, as long as its source code is provided under the same license (GPL). Now, I do not understand how it is consistent with Section 2b:
You must cause any work that you distribute or publish, that in whole or in part contains or is derived from the Program or any part thereof, to be licensed as a whole at no charge to all third parties under the terms of this License.
I don't get it. Does not this clause prohibit selling the software? If not, then what exactly does it trying to say?
EDIT: Following the answers below, I wish to note that answering a question "everyone says that GPL says A but this part of GPL says B, how is it?" with "everyone says A so GPL must mean A" is not an answer.