You are not supposed to modify the code of the module you purchased anyway (lets call it module A).

Here is my understanding :

1/ You need to make your modifications in a separate module (module B) that depends/inherits from module A.

2/ you can sell your module B... but the end user will need module A to use module B, so he will also need to purchase module A from the author of module A (which is not you obviously, as you are not allowed to distribute module A, as highlighted by odoo proprietary licence).