You need to have the following (not limited to). They can be achieved by integrating to other systems as well.
1. Catalog system (including catalog maintenance, update, delete)
2. Inventory system
3. Payment system (including operations for authorization, debit etc)
4. Order fulfillment system
Optionally you can have integration with reviews and ratings, store finder functionality, fraud detection mechanisms, Merchandizing tools, search functionality management, personalization components, order tracking functionality etc. Obviously you need the IT infrastructure to support each of these features :-)
Hope this helps.
Keep posting your updates / questions.