Several issues here, I feel.
Firstly, claims that any 'deal' to refuse returns are underhanded and involve Microsoft are quite correct. That's bad practice according to any criteria of analysis, including a legal one.
Secondly, any implication that Microsoft would be justified in believing that an OEM or an Educational license is only valid for the original purchaser is also quite justified. It's reasonable to expect that a greatly reduced price restricted license would have such a restriction. Unfortunately, Microsoft, to my knowledge, does not explixitly state that this is the case in the license agreement, and I'd argue that for it to be valid for an unopened product, indication of it should be included on the packaging!
Thirdly, it is NOT the case that an OEM Windows is only available with a new PC! An OEM Windows can be purchased with components as well, and in the event that it is, then the license is tied to that COMPONENT, not to the system. I've personally got a home built household system which uses an OEM Windows XP which was purchased with the hard drive installed. When I later decided to replace motherboard, processor and display card in it, to improve its games capability, I rang Microsoft first, knowing that a reinstall was in order so that I could be sure of the system booring to Windows afterwards. Here's the response:
MS dude: No sir, you cannot reuse that OEM Windows XP, because if you replace the motherboard it will be a new system.
Me: No, it will not be. In any case, the license is tied to the hard drive, not to the motherboard.
MS dude: No sir, that's not the way it works.
Me: Yes sir, I'm afraid it is. Put your supervisor on the line please.
Supervisor: No sir, you cannot use that OEM Windows with a new motherboard. It's a new system.
Me: Yes, I can. The license was purchased with the hard drive, not with a prebuilt system. The computer is, and will be, the same machine. It has the same box, the same monitor, the same peripherals connected to it, and will have the same programs and data installed to it. All I'm doing is giving it an engine overhaul. If I fit a new engine to my car it's still the same car!
(I then quoted the relevent details from the license agreement, together with the relevent sales receipt numbers.
Supervisor: OK then sir, when you reinstall, please call our centre here by phone to reactivate, and we will ensure that the process takes place.
(I then got his name, for further reference, along with his personal assurance that written communication and the furnishing of proof would not be required.)
If you have a genuine case to argue, pursue it. They do NOT want to spend large sums of money prosecuting cases which could only end up making them look foolish. But if you let them con you, they will