Question about Microsoft Windows 10 Pro

I see that a member of the community, Anonymous, is selling this same software second hand. If this software has been installed, on an Anonymous computer, will it be recognised on ANOTHER pc? It is my understanding that an OEM version only works with the computer it is installed on and cannot be migrated to another computer. If I'm right, why would they allow such a product to be offered for sale (usable only on Anonymous' PC)? In case I'm wrong, please let me know, as I'm interested in the price for win10pro64bits.