This is what Microsoft website OEM FAQ says:
Q. My customer bought a new PC and wants to move their OEM software from
the old PC to the new one. Can't they do whatever they want with the
software?
A. The OEM software is licensed with the computer system on which it was
originally installed and is tied to that original machine. OEM licenses
are single-use licenses that cannot be installed on more than one
computer system even if the original machine is no longer in use. The
end user license agreement (EULA) accepted by the customer before they
use the software, states that the license may not be shared, transferred
to or used concurrently on different computers. The System Builder is
required to provide end-user support for the Windows license. A System
Builder can not support a license that has been moved from a PC they
manufactured to one that they did not — this is a fundamental reason why
OEM System Builder licenses can't be transferred.
Q. Can my customers transfer or sell their OEM software licenses?
A. After an OEM software license has been installed on a PC, the license
may not be installed on or transferred to another PC. However, the
entire PC may be transferred to another end user along with the software
license rights. When transferring the PC to the new end user the
software media, manuals (if applicable) and certificate of authenticity
label must be included. It is also advisable to include the original
purchase invoice or receipt. The original end user cannot keep any
copies of the software.
The end user license agreement (EULA) is granted to the end user by the
System Builder and relates to the license on the PC with which it was
originally distributed. Because the System Builder is required to
support the license on that original PC, a System Builder can not
support a license that has been moved from a PC they manufactured to one
that they did not. This is one of the key reasons why an OEM System
Builder license can’t be transferred. For more information, click here.
Show them a copy of this message and see what their reaction is. They
want to sell you a software and so they will not volunteer information
unless you ask them specifically. This is pure business.