You would be surprised what some small shops get away with, I've seen some extremely unethical practices discussed in back rooms of some of these places, in order for the shop to spend zero dollars on certain things, they charge the customer full price for an installation and activation. That's why I know so much about how it is done. It's 100% profit for them. Most of the shops I was familiar with that operated this way are no longer there, I have never seen them again... And maybe I saw them do something that made me so mad that I actually told somebody about it.

So I am pleased that the OP asking in this forum, it is better to ask, and to KNOW what you can and cannot do.

One shop I know of, that sells used PCs, they always include a licensed and activated version of the OS, with install disk. They may have gotten the Machines for nothing, but they always buy new OSes for them. People bring their older systems in and trade them for new ones, they get a decent discount if they leave the old machine. They just reformat the HD's, install a new OS into it, its not as much profit but it's a lot better, because the new user has some kind of warranty.

SO if you are gonna sell used equipment, make sure the Motherboards are in good shape, no Electrolytic caps about to Burst open, no dust, case id clean, and if the original owner did not give you all of the original disks and manuals, you will have to buy a new OS for it.

Actually, if XP is still being sold to Shops, you should buy as many copies as you can.

But I don't think you can, here is the page for the System Builder Program:

OEM System Builder Licensing