According to supercells terms accounts are non transferable. So selling an account would be a breach of terms and could very well get it banned.
"The following restrictions apply to the use of the Service:
You shall not (or attempt to) purchase, sell, rent or give away your Account, create an Account using a false identity or information, or on behalf of someone other than yourself; You shall not use the Service if you have previously been removed by Supercell, or previously been banned from playing any Supercell game."