There’s a lot of people saying not to pay for windows or not to use eBay or to download from Microsoft directly or to use massgrave instead but there are very few possibly no responses explaining why that’s a better option than buying from eBay.
First things first, and I know this is gonna get some people responding with the ol’ “nuh-uh”, but there’s compromised counterfeit install media for sale. You most likely don’t know how to tell the good from the bad. The bad can also look very, very good.
If you want to know you’re getting the good stuff straight from Microsoft, use uupdump to create an updated install media for whatever sku(version of windows) you want and use rufus to make the usb installer (you’ll need to be inside a windows vm or on your old version of windows). The reason I say use Rufus is because it gives you the opportunity to turn off some stuff like requiring a Microsoft account in the 11 install process.
Once that’s done and installed, you can activate however you like, either with massgrave or with a key for your sku purchased from Microsoft directly.