Please take not that wordpress.COM blogs cannot be equipped for e-commerce transactions. We can have only a PayPal donation button and/or a contact form on our blogs. We cannot retail or resell anything we do not the creators or providers of. This entry describes the types of blogs allowed and not allowed at wordpress.COM http://wordpress.com/types-of-blogs/
If you want a blog that can be equipped for e-commerce then you will need to hire a web host and get a wordpress.ORG self hosting install.
If you stay at wordpress. COM then these apply:
PayPal donation button