Ghost is a platform to create, publish, and email content to a member list, as well as offer subscription services for the content you create.
You could definitely use Ghost to market a physical product, but we're not really an ecommerce store—we won't provide inventory management tools, for example, and we don't have a built-in shopping cart. However, we do have some ecommerce integrations listed here that can help you sell products on your site along with your publication. We also have easy ways for you to embed things like PayPal's "Buy Now" buttons into your site.
Overall, you can sell products on your Ghost site if you so choose, but the main idea behind Ghost is publishing. There aren't any extra fees to set up your Ghost site to be a marketplace unless you'd like to use a custom theme or custom integrations. You can check out all of our plan features on our pricing page.