David, age 35, is an avid camper. He has just bought a new tent in preparation for a camping trip. He wants to sell his old tent, so he advertises it on an internet resale site at a price of $250. Erik, age 20, is looking for a used tent, and sends David an e-mail proposing to buy the tent at that price.
Question: List and explain the four requirements of a valid contract. Which elements of the scenario above correspond to the requirements?