Look, if you buy it from a non-trusted source, nothing can be done. Microsoft won't sue anyone of you. It's up to you to claim a legal action against the person. However, if you bought from a recognized reseller and the Windows gets out be illegal (pirated) (because Ebay once sold pirated and non-genuine Windows copies) you should can directly contact Microsoft about it and they would begin a legal crackdown.
Furthermore, you should never buy a used license or buy Windows from an anonymous person. Purchase it from official Microsoft partners.