Meaning of Enterprise Resource Planning regarding a E-Commerce site

Often, I was asked why a resource management system is suitable to be framed in an online store … In fact, operation is simple:

Regular client can access a restricted area with a password where you find prices and personalized offers will be inserted into shopping cart products they intend to buy and send the command directly. Being a regular customer, it will automatically apply the terms agreed in the “General Conditions of Purchase”, established the first contract. One click and the order is made​​.

What do you say? Generaly, online stores have a default function ERP, or separate store itself? (So that we can speak of a special software for this purpose)