Windows 10 Pro retail version - will not activate

Ge ShowKeyPlus from the Microsoft Store.

After you run it select Check Edition. Enter your key to see what edition it is.

Azucho98 said:

Purchased retail Windows 10 Pro kit on ebay from a reputable seller

"Reputable" by who's definition. What you say, in itself, rings alarm bells loud and clear.

They sold a bunch of them and had all positive feedback including ones sold recently. The kit was sealed, and the label was over the USB drive that contains the software.

Absolutely no guarantee whatsoever. Sorry.

The graphic of the box label, can usually pick out the fake reproduced labels (faded, minor imperfections, missing components)

I guess it is possible the kit was opened, the key used previously, then resealed and sold. If that were the case, wouldn't activation tell me the key was already in use?

Nope. Either of 3 things can happen
1) the key is an outright fake (made up numbers and letters that do not match the algorithm for a key for that version - usually rejected at input
2) a re-used key that will get rejected after installation is complete and activation attempted
3) wrong key type for wrong version (like under new conditions for Win 7/8.x keys no longer accepted for Win 10/11)

I'm thinking either 1) or 3), more likely 3) as has been suggested already

Buying (any) software keys off market sites are fraught with dangers. Looks like you'll have to go for a legit key from a legit site/seller (cf. a "reputable" site/seller)

Azucho98 said:

So Showkey says the key is OEM which is very strange because the key card itself says "retail" in the lower right-hand corner. How can a retail box kit contain an OEM key? It certainly looks like legit Microsoft software - box has the "genuine" indicators on it but who knows.

Anyone can make boxes and flash drives. That is OK with Microsoft. The only thing that matters is whether the Product Key is genuine and unused.

If it won't activate and you can't get your money back then your only option is to call Microsoft technical support for help.