In reply to MeMeMe:
WordPress will have a plugin for this.
There will be a tried and tested solution that does what you want but it might not be exactly what you want and it might not be free.
I assume you'd have to get a hosting deal rather than run it on WordPress.com.
WooCommerce* is the "default" for WordPress ecommerce.
Magento* is the "default" for "proper" ecommerce
*it would be interesting to hear opinion of others on ukc on WooCommerce/Magento for SME ecommerce where you can't really justify bespoke development.
Post edited at 10:11