As far as I know, it is legal to sell an installation or source of software (that is GPL licensed) for any price you wish, under certain conditions among which are:
- You must supply the source of all software.
- You may not impose any other license on that software.
Some distro's build their own software and do not license it under the GPL (SuSe is an example), but many distro's are free to sell/give away/modify/etc.
For more info see the GPL license:
FSF License page
Ensure that any software you want to sell, is under a free (as in speech) license that allows you to.
P.S. I believe the BSD license allows you to sell aswell under easier conditions.
P.P.S. IANAL (I Am Not A Lawyer)