Selling skins sounds like a bad idea to me. If the trend catches on, we'll be having a lot less downloads available and a lot more people mad when they find out their pay-for-play skin/model is being downloaded for free on someone else's site.
I put a boatload of work into my skin and I wouldn't dare charge money for it. Two reasons behind this:
1: I download other people's works for free and I want people to download mine for free as well. It's a shared thing. You take, you give (if you can). Charging for it? It better be damned good, or a completely different game or something.
2: I make the skins for myself first, and if I like them it's all the sweeter if other people like them, too.
People who make this stuff in the industry do it for a living, and sometimes they don't even like what they're making. They're paid to do it, so they do it for the money. If they like what they're working on then it only makes their job better. Look at Counter Strike... Sure, you could pay for the retail version, but isn't it still available for free? I dunno. I could be wrong there, I don't play Half-Life games.
Anyway, that's my thought on the subject. I don't think people should charge for mods to a game.