According to the associated licence summary, when you purchase a regular licence
You are licensed to use the Item to create one single End Product for yourself or for one client (a “single application”), and the End Product can be distributed for Free.
You can’t re-distribute the Item [...] with source files.
So putting it on github strikes me ...
If it is documentation, you can e.g. write a book on emacs, or GNU MP (one example among many), Python or Perl, keep it's source closed and sell it. This is quite common, just take a peek at O'Reilly's offers, some are open source, but not all. Note the software described is under different licenses.
If I use a compiled program [...] which is under a license that everyone has the rights to use and sell (Eg: GPL) [...] and then I want to develop an extended feature for it [...] to be sold for money, to be available only to the person that paid for this premium feature - Must I release this premium feature also as GPL?
If your work is a derivative of the ...