“System Builder” means an original equipment manufacturer, an assembler, a refurbisher, or....
Even so, I would have thought 'an assembler' would have allowed me to qualify. (I have the receipt for the parts.)
It's academic really, every big seller seems to be selling OEM to the public. One just says that the packaging is spartan, and that's how they save the money. $100 plus, for a coloured box! I don't think so.
It really is up to MS to regulate their outlets. I used to build in the DOS and early Windows days, and had to get my soft from one of several major suppliers that would not sell to the public. Indeed, some only dealt with limited companies. I never saw OEM DOS for sale in the shops - not that there were many of those back then.