You are to develop an online auction site, with selling, bidding, and buying services. The buying service should allow the users to find an item, bid for it and/or buy it, and pay for it.

Consider the online auction site described above. Suppose you want to employ the Publish-Subscribe (also known as Observer) design pattern in your design solution for the auction site above. Which classes should implement the Publisher interface? Which classes should implement the Subscriber interface? Explain your answer.