WordPress.com is not set up to be a full ecommerce site, but you do have the ability to add a Paypal button to your blog.
You might want to look at our Types of Blogs page to see what kinds of blogs fall into the 'not allowed' category.
All WordPress.com blogs come with 3GB of storage space for free. If you need more space than that, you can purchase the Space Upgrade to get an additional 10, 25, 50, 100, or 200 gigabytes of storage space.
Does that answer your questions?