It's my pleasure, you are welcome to use the image. You could even mention that the coin is on sale with this topic details 
I have actually long thought about putting a section to the site for coin sales. People could make sell listings there in a centralized place, much more organized then on this forum. then I could direct the buyers to the sellers, and take a small cut. Do you think this would be successful?
I believe you are completely right with this idea.
You already made a good product that people use and everyone who's interested in coins have.
If your book would be downloadable with email opt-in box, you could have a worldwide Casascius fans email list.
I believe you mentioned that you will soon launch V.2 . So we all would need to renew..?
After you have the mailing list and time to work with it, you could help sellers and buyers.
At first with at sale email promotions, later with an actual website.
That's my advice;)