I'd love for Users to create sigs and sell them to people, I'm also wanting to sell the actual privilage of having a sig too. I'm still looking for a way to sell things like BBcode to users, although I spent yesterday beating the display.template.php file into letting me sell icons which appear in the user info section of posts.
I'd love to know if anyone has already done anything like this, especially the BBCode one
Only problem I see with selling sigs, is that people might try to steal it. You would to somehow restrict the ability to right-click and save. On top of that the image would need to be reduced in size or stamped with something so people can't use it right away.
And I mean reduced in size, as in that even if you right-click and save the image will still be that small. I've seen some sites that have reduced images, but if you right-click and save, you will get the full size version. I think it was some html that reduced the image size, but still had your browser upload the full image size.
Just make sure that there are alot of security features added so people can't steal the images, either through right-click, print screen or by it being cached in thier browser. Those methods I listed should keep that from happening.