LEGO wrote:
As far as you distribute the source code and any changes you've made
under GPL nothing forbids you to sell it.
But it does forbid you from forbidding those who have bought it from you
giving it away for free without paying you anything, and they also have
to be able to get the source and give the source away, for free, without
paying you anything:
http://www.gnu.org/licenses/gpl-faq.html#DoesTheGPLAllowRequireFee
"Does the GPL allow me to require that anyone who receives the software
must pay me a fee and/or notify me?
No. In fact, a requirement like that would make the program non-free. If
people have to pay when they get a copy of a program, or if they have to
notify anyone in particular, then the program is not free. See the
definition of free software.
The GPL is a free software license, and therefore it permits people to
use and even redistribute the software without being required to pay
anyone a fee for doing so."
and
http://www.gnu.org/licenses/gpl-faq.html#DoesTheGPLRequireAvailabilityToPublic
"If I distribute GPL'd software for a fee, am I required to also make
it available to the public without a charge?
No. However, if someone pays your fee and gets a copy, the GPL gives
them the freedom to release it to the public, with or without a fee. For
example, someone could pay your fee, and then put her copy on a web site
for the general public."
so if you're going to charge money for it, you might want to charge
enough money for it that you won't get somebody buying it and giving it
away just because, for example, they don't like the fact that somebody's
charging money for a modified version of Ethereal....