Heyyo,
It's in m$'s licence agreement that their products are not for resale. So yeah, it's actually illegal to buy windows xp, and sell it off for a higher price on things like ebay. In your case it's the same man. If anything hand them out to buds, maybe get them to pay ya $5, ya know? under the counter deals. M$ won't care for that. But selling em' off high price, WITHOUT LICENCES, and charging good cash? oh dude you'd feel m$'s wrath.
Btw, have you tried doing a search on e-bay? they're all by online retailers, maybe 1-3 users trying to actually sell off. Brave little ones aren't they?