Check out my signature links to find a free tutorial on creating a JSF portlet.
As far as EJB 3 goes, your portal has to support it. For example, IBM has no intention of ever supporting EJB 3.0 in a full release, so using EJB 3.0 with WebSphere Portal is not a good idea. Other vendors may have different ideas.
A portal helps with page aggregation, user management and content management. If your ecommerce site could benefit from this, then by all means, use the portal!