If you want to go to the trouble of providing the installation media for the Windows version that works (is that even legal?), I could see selling it as a bundle at a little higher price like $10 more, IF you go to the trouble to make sure that it does actually install without activation issues which is sort of a wash IMO, to bother doing that for $10 more, so I might just include it without the extra charge.
I could also see you losing sales to someone else selling the same board $10 cheaper without that, or gaining sales selling same board with that but at no higher charge. If it's legal, let the market decide?
The thing is, who buys OEM boards unless it's a direct replacement for what their OEM system already had, and then they already have a license for that system/motherboard? Those people "might" still pay a premium for a drop in replacement because it saves them the trouble of a fresh OS install, and they already have the license, but price it that high and again you aren't as competitive with other sellers.
Of course there are some niche applications where someone good at tech, recognizes their needs and wants a cheap board to fill them, so it's not a repair-replacement. I too have done that years ago, but only because they were dirt cheap and not a proprietary form factor.
Lastly if the board was sold in a system, the buyer typically intends to use the system, so not even sure just how much these used pull motherboards are worth, certainly something if not so old that nobody needs them, but I also used to buy brand new OEM boards and certainly something with no years of use is worth more. If you're parting out new systems, I'd think that's going to take a lot longer and/or work to make money on all lthe parts rather than mix and match, selling them as whole, working systems.