The answer is not so clear without access to the precise agreement
the seller has with Microsoft.
Remember there are actually two agreements involved.
The agreement between Microsoft and the seller.
The agreement between the seller and the end user.
If the seller violates the agreement with Microsoft then Microsoft
could deny based on an illegal sale.
However if Microsoft did deny, it would most likely come back
because the buyer does not always understand.
Just because Microsoft (or any other business) has a right to
something does not always mean they will always exercise that
right.
Ah, I see. But that's three agreements, as the EULA I have is a
contract with Microsoft, not the seller I bought the OS directly
from. If my EULA's not actually valid (because Microsoft never
actually agreed to it), then I'd be plenty mad at the people who sold
it to me and at Microsoft for having no public statement (AFAICT)
about these arrangements, which are fairly common.
FWIW, I bought my OS from a reputable hardware dealer (NewEgg) along
with a case, power supply, mainboard, CPU, HDD, floppy, and video
card. I bought a DIMM from another company at the same time. Since
then I have had two HDDs fail (both RMAed easily, one with NewEgg and
one with the manufacturer, Western Digital) and I added another HDD
to my machine. These changes have not affected my ability to
activate online. I was never asked to reactivate, but I have
reinstalled and activated a couple of times. I don't expect to be
able to change my mainboard, but I do expect to upgrade the CPU and
add more memory without Microsoft ruling that I'm trying to use my OS
on a different computer.
I tried to find a leaked agreement between Microsoft and the hardware
retailers who distribute these OEM versions, but I can't come up with
anything. I know that Microsoft's settlement of the big DoJ case
involved remedies regarding what they can and can't do WRT the
preloading of their OS's by OEMs and what they can and can't do WRT
contracting with OEMs. I suspect that Microsoft does not have a
choice about whether or not to let this practice continue. But
IANAL, and I can't wade through it all.
The shady hardware dealers who are selling these things with only a
mouse or a cheap NIC are not helping matters any.