You may be right, but there are still people with valid questions and I don't want to bar them. Actually, I know you're right, because I've changed the wording of my "Not for Sale" about 10 times.
T
Your Not For Sale sign is on the home page. People probably wouldnt pay attention to your message at that point, if they see it, instead likely quickly clicking a gallery link in the menu instead.
You are better off writing Not For Sale against every poster, but if that is too hard, instead of making the Contact Us in the left nav menu a mailto link, instead link it to a newly created Contact Us page on it stating clearly the posters are not for sale with the mailto link there instead. That way the user will clearly see it is not for sale before emailing you, where currently they likely never see or remember such a message.