That's for an OEM version of the software, not an OEM version of the software that comes with hardware. My last gig I dealt with compliance, I do the same thing at my current job. I've been audited by Microsoft when I worked at an MSP. When they sell you piece of hardware with a windows key tied to it, its logically tied to that piece of hardware and is not transferable even if it hasn't actually been installed or used. OEM can not be transferred , Full retail gets a single 1 time transfer.
The closest thing to what you are doing is more like this scenario:
What happens if I change my motherboard?As it pertains to the OEM licenses this will invalidate the Windows 10 upgrade license because it will no longer have a previous base qualifying license which is required for the free upgrade. You will then have to purchase a full retail Windows 10 license. If the base qualifying license (Windows 7 or Windows 8.1) was a full retail version, then yes, you can transfer it.
Since you are buying a PC logically tied to an OEM Win10 Key, you cannot transfer it.
Again, all of that is moot since personal end users aren't audited by Microsoft. Your method works, you can't be caught, but its still not kosher