Also if these are Trade Ins you have an obligation to your customers to protect their data as you can be held responsible for any loss if things go wrong.
It's far cheaper to just install a new HDD and provide a copy of Linux than to have to appear in court for not acting professionally.
If all you are attempting to do is retain the Windows OS and you are working for/as a charity you can get permission from MS to load Windows onto these computers. But if you sell them loaded with Windows and without a copy of the Install CD you are in breach of MS EULA.