The obvious requirement is an on-line store, or commerce-enabled website where goods or services can be described and selected.
While it is possible to run an on-line store without accepting on-line payments, this is cumbersome and rarely successful. Accepting on-line payments is therefore essential- and at the moment this means credit cards.
ok, so you've made your sale and now you've got to deliver the goods. Just package them up and ship them off. Easy - except what happens if you get dozens or hundreds of orders a day?
Support, Complaints, Returns - the biggest complaint about E-commerce is the poor level of customer service in the event of problems.
Even if you do everything else right, without successful promotion your on-line business will fail.