I don't really like the idea of monetising avatars, I think we need a proper avatar system (one where you can actually earn them) before we start handing them out left right and centre.
The specific trouble with selling them is that people can then rightly keep hold of them forever, which means any future system would be polluted by these custom avatars.
If people are really tied to the idea of selling stuff, might I suggest avatars are leased instead?