Gwyn wrote:
One small point - it isn't a * lens.

I have the Sigma 10-20, and whilst I don't use it much I am not sure I want to sell it until I have really played with it and decided if it is for me or not.
Unless you really need the money hang on to the lens and spend some time playing with it. Get up close to buildings and see what you get. If after playing with it some more you are sure you will never use it then sell it.
It isn't meant to be a lens which lives on the camera, it is more specialised than that.