Now I don't want to lose Windows because I may sell it.
You probably wanted to start a new thread for this, but it does not matter.
My suggestion is to get all the data you need off it, wipe it, and state in your advertisements that since the buyer will have to load their own OS*, you will discount it a little (it does not have to be much, a Jackson or two will satisfy most people that they are 'getting a deal').
But! Offer to help them with this by providing a free, privacy-improving (a big feature point post-Snowden), intuitive and customizable OS at no extra charge (I can supply a CD or cheap thumb drive), in which case they can come to the GLUG to get help installing it and being introduced to it.
If they are not interested in this, remind them they can always buy whatever proprietary OS they want (probably Windows, although it is no longer a given that everybody wants the latest version, since Microsoft has made so many changes so rapidly, some of them pretty dubious - people seem to like still using 7, or even XP [which removes even the missing-the-point "it is supported" argument for proprietary software], so it might even be an advantage not selling it with Windows preloaded) and install it themselves, with plenty of online resources helping them to do that.