I would never buy software from eBay.
Warning:
You do not buy a Product Key by itself, as they are normally stolen/pirated, Volume Licenses which are not for private use, or MSDN keys which are not to be sold by the original subscription holder.
Make sure that you buy the complete sealed package (DVD, license)
https://www.microsoft.com/en-us/howtotell/Shop.aspx
Read Section: Beware of Product Keys sold separately.