The Code is released under GNU GPL v2, which means any one can edit the code as he wishes, and any can even sell free software ! .
but he has to distribute the GNU GPL V2, license with it.

The logo ( Chevereto NB) is licensed under CC license, and cannot be sold.

how ever as the Author of Chevereto NB has mentioned in the license_agreement.txt that

You can't sell Chevereto or any modified version. You can't
sell the distribution of Chevereto or ANY modified versions.
I think it means that ,
users cannot sell the / modified script as script alone and claim that they have coded it,
but they can sell the site which uses the script/modified script , as its still under GPL v2 license ( they have to give the Copy of GPLv2 off course ).