If the key is used, you can not use it?
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.
Let's give you a good in-store price of $250, knock 10% off for it being used, then offer you free shipping. 45 bitcoins.
....and 10% off 'Store-Price' for a used/activated
Windows License ? are you high ?