Mountain Lion was never free, other than coming installed on new Macs for about a year, and perhaps a free update at changeover (Lion installed, with the new Mountain Lion a free upgrade ?)
Anyway, it still sells for the same $20 at the Apple web store.
The "purchase" is to get you a redemption code to download ML in the App Store.
I expect that redemption code will ONLY work for your own AppleID account, and for that reason, probably not transferable to anyone.
You don't have a legal way to sell the license to someone else.
And, if Mountain Lion is in your Purchases in App Store, you can always download it at no further charge.