1. You may copy and distribute verbatim copies of the Program's source code as you receive it, in any medium, provided that you conspicuously and appropriately publish on each copy an appropriate copyright notice and disclaimer of warranty; keep intact all the notices that refer to this License and to the absence of any warranty; and give any other recipients of the Program a copy of this License along with the Program.
You may charge a fee for the physical act of transferring a copy, and you may at your option offer warranty protection in exchange for a fee.
Noone is stopping anyone else from selling it, but I think you're missing the point. If they're literally selling YOUR copy, only idiots would buy it because they can always come get it from YOU for free.
IF, on the other hand, the person doing the selling has made significant modifications to the code by porting, enhancing, or warranting it, etc, they might actually find buyers. If that's the case though, why would you care? They obviously put a significant amount of work into improving on your original concept and created a product people are willing to pay for. You still credit for being the building block though via your copyright notice in your code.
The perrenial example is Linux. Linux is free at kernel.org and pretty much all of the tools can be had for free from around the net, yet people still buy a distribution from Red Hat. That's because, while Red Hat didn't MAKE most of what they're selling, they did go to the trouble of collecting it all into a nice form and making it easy to install, supported, and easy to update. You pay them for the convenience they added, not for someone else's code. There are more ways to add value to a product than just making it, and that's what other people would be able to sell using your code as a base - the value THEY add. If they add no value, people will just come get your code for free.
Additional note in reference to icrf:
Noone is obligated to accept the GPL or place it on derivative works. However, if something is licensed under the GPL, they can't redistribute the code or modified versions of the code - since the GPL covers distribution - until they accept the terms of the GPL. They can still USE the code because the GPL doesn't cover usage.