A Microsoft Software License that uses Product Activation is worthless once activated by the original purchaser.
You would be buying a guarunteed copy of Windows (as in not worth any more than the price of the DVD itself) and an 'iffy' License/Key (perhaps fails activation)....and if you need to take steps to circumvent PROPER activation methods, you might as well just use pirated software from the start.
....and 10% off 'Store-Price' for a used/activated Windows License ? are you high ?