For what it is worth. This is just my opinion and there are going to be several out there that will disagree with me about it.
It is not legal for you to purchase a system builder copy of Windows and upgrade someone's system with it. Having said that I have done so for years and not had Microsoft, the FBI, the BSA, the CIA, or the KKK come and kick my door in.
Microsoft also turns a blind eye to places like discountmountain who often sell surplus copies of Windows that have been branded by an OEM. Dell stamps right on the disk For use only with a Dell Computer, yet they must have sold the disks to discountmountain in the first place. So long as Microsoft is paid for the copy of windows, which Dell already did, they really don't care what is done with it and are understanding of the slightly under the table liquidation of surplus stock in order to offset the loss of purchasing it and not being able to sell it.
In the past Microsoft has told System Builders that OEM copies could be used to install over illegal warez copies and then they have flipped flopped and said you cannot. In the past you had to sell hardware with single copies of Windows. Got many a bag of old screws tossed in with a copy of Windows. Now you don't need to do that. Again so long as Microsoft gets paid for the copy you are selling your fine. I've never seen anyone get busted for selling OEM as upgrades. Plenty of pirates busted for counterfeiting but not selling something legally purchased from Microsoft and sold.
Microsoft has bigger fish to fry then to bug someone for selling a legal copy(not Chinese counterfeit) copy of Windows.
My opinion and have the board will disagree with me about it so it is your choice on what to do. No one will consider it bad if your are conservative and stick with retail copies.