I am looking at the creation of a large ecommerce portal that can be used to select items, update quantity, place orders, and ask the shipper to pick up items. A lot of this is transactional in nature like billing, payment, shipping etc.
RPC approaches seem to be the traditional way of doing such distributed processing.
I am new to REST, and I know the basics. What scenarios can REST be used in a large transactional ecommerce portal?
Yes REST would definitely benefit you.
I think you should check this doc by Alexandros Marinos http://docs.google.com/View?id=ddffwdq5_2csz22wfd&pageview=1&hgd=1, he explains in details of using RESTful transaction.
Yes REST could definitely be used for this type of application.
However, be careful with the word transaction. If you mean transaction as in a "database transaction that can be automatically rolled back" then it is important to realize that those transactions should sit behind the REST interface.
If by transaction you mean "a series of steps that a user can choose to complete or cancel" then yes, REST can model those transactions as additional resources.
I asked a question on REST a few days ago that yielded excellent feedback. Maybe some of it benefits you as well.