Online Auction: A service in which auction users or participants sell or bid for products via the Internet. This is the method through which many items on eBay are sold.
PayPal: An online payments system that supports online money transfers and serves as an electronic alternative to traditional paper methods like checks and money orders. PayPal was once owned by eBay and was commonly used for transactions on the site.
Consumer-to-Consumer (C2C): A business model that facilitates commerce between private individuals. This is the type of commerce most commonly seen on eBay.