Thread:Project:Support desk/Selling Media wiki skins?/reply

Like the rest of MediaWiki, MonoBook is licensed under the GPL. If you are going to distribute a skin based on MonoBook, it will have to be licensed under the GPL. Thus, those who purchase the skin would also be permitted to redistribute your skin under the GPL, for free or for profit.

It's less clear whether a skin created from scratch would also have to be GPL-licensed.