I don't think you can buy W7 from MS anymore. eBay should be fine if you can find sealed, retail box, retail version. Make sure you read carefully. If it doesn't say full and retail, it probably isn't.
The real perk of getting the retail version is that you're buying a one user license that can be transferred. If you build or come across a nice hardware later, you can move it.
The system builder version becomes affixed to the machine you install it on and can not be moved.
Well, someone could clarify this: I'm facing only the possibility of a "situation" in the next couple months.
I purchased an OEM "white-box" (it's an envelope now) 64-bit Pro SP1 around September 2 from Newegg, paying the usual price (~$120 more or less). I wouldn't know how those cardboard envelopes and discs appeared on the market or when, but they are quite genuine.
So unless MS's policies about hardware changes have changed for Windows 7, you would re-activate by telephone with the proper answer about "only one computer."
I'm building a system. In this regard, I make purchases in excess of requirements, because it can facilitate convenience, and I can always use the leftovers in unforeseen applications. So I bought a license to Win7SP1Pro, and a license for Win10 -- also in a similar white-cardboard envelope.
I bought my motherboard because it had certain features, particularly 12-phase-power-design which I thought was "top-tier" because I'd forgotten we've gone through a couple generations of CPUs. "Top tier" now features 16-phase.
I am thinking about shelling out for a different Z170 motherboard sometime in the future. Some time after that, I may start looking for a Skylake or Kaby processor, and swap one processor assignment. I can't say, I don't know.
If I have to pay for another Windows license and installation, I will be peeved. But I can see how I could also avoid that, too, so that one of the systems only runs Win 7, and the other runs 10. I was going to create a dual-boot affair, but that's something I can sacrifice.