Consider the online auction site described below:

Consider an online auction site, such as eBay.com, with selling, bidding, and buying services. Assume that you are a buyer, you have placed a bid for an item, and you just received a notification that the bidding process is closed and you won it. Write a single use case that represents the subsequent process of purchasing the item with a credit card. Assume the business model where the funds are immediately transferred to the sellers account, without waiting for the buyer to confirm the receipt of the goods. Also, only the seller is charged selling fees. Start from the point where you are already logged in the system and consider only what happens during a single sitting at the computer terminal. (Unless otherwise specified, use cases are normally considered only for the activities that span a single sitting.) List also some alternate scenarios.

Suppose you want to employ the Publish-Subscribe (also known as Observer) design pattern in your design solution for this problem. Which classes should implement the Publisher interface? Which classes should implement the Subscriber interface? Explain your answer. (Note: You can introduce new classes or additional methods on the existing classes if you feel it necessary for solution).