Edited my above post, second part is why and, why would someone go through the [albeit small] process to put a pic in their sig/avatar if they were just going to turn around and sell for profit? A sig almost confirms that they won't. Like someone else said, not fool proof, but a bigger chance than if you just say you like them.
Well if I wanted to make some money I think it would be pretty easy to write some names in my sig and say they're my dream villagers ... I personally think it's more effort to write a small paragraph on why you want them. It would be nice if things were done that way, I have seen a few threads like that. Or simply just first come first serve. A silly little signature really doesn't prove anything.