When you bought a physical item in a store you have a right to resell it be that a computer or a disk.

Even if you can resell a key as part of selling a copy of windows you paid for you can't sell your employers property, can't sell part of a contract, can't sell more than you actually paid for.

If the company purchased the right to have n machines at their company run windows and each key can be activated n * x times nobody sold you the right to sell n * x copies of windows to everyone on the internet.

Like it or not those agreements aren't sales and the keys aren't a physical item you have purchased they are just a technical measure to constrain users from sharing a single ISO file to their million closest friends.

Even if true, I rather doubt that extends to an employee selling access to their employer's volume licensing key (it seems clear that's what's happening here)

Source on this? I think that only applies to retail licenses.