It definitely falls into the category that the reseller is illegally selling keys...Skip it. End users aren't eligible to receive key-only licenses, from what I'm aware.
This would apply even if the reseller bought a technet/MSDN subscription, and tried selling the keys individually...Those are strictly for individual testing/evaluation, NOT for resale. I'd figure that if M$ found out what was going on, it's a simple matter I'd imagine to simply invalidate the keys and make it impossible to update the system anymore--Along with the other drawbacks an illegal/invalidated key would inherit.
Plus, the keys could be counterfeits/fakes as well.