Where can I get a copy of Mountain Lion that I can resell with my Apple hardware?
It's not at all clear that Apple will endorse the transference of an App Store account and it's clear they don't endorse transferring parts of the purchase history. Legally, you might be able to get away with this in certain parts of the world, but it's clearly not something Apple is keen to point out how to accomplish this or has built in a "gift my license" functionality to their stores at present.

Why not include a gift card for the App Store in question or some other cash equivalent with the sale?