If your company have volume licensing then you know how, normally enterprise will using volume licensing.
Volume Licensing - one key for all.

Microsoft use based on trust, so even you only buy for 200 units but you get activation of at least 2500 times as minimum.
This activation will get increase based on total quantities you purchase.

Those seller is basically illegally selling the valid key from Volume Licensing. Thus it is valid forever unless you get suspect by Microsoft.

This post has been edited by Andrewtst: Jul 20 2020, 12:42 PM

Basically volume license is upgrade license, you need to have base license, for example oem original license. This is the proper way and legal way.

Price wise is depends your account level. Different level different price. For example my work company cost each half the price of box set.

You need to check with certified software retailer company, they will assist you, you cannot simply buy at outside shop.

Example selling this kind of license include SoftwareOne or Crayon.