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PayPal. When you go to send money to someone, you are asked what kind of transaction it is, and the choices include Service, Goods - Auction, Goods - Non-Auction, and Quasi-cash. If you have an account, and someone else has an account, and you want to sell them something, you can pay through PayPal, no eBay required! Pay Pal is completely independent of eBay. In fact, eBay has their own service called BillPoint, that competes with PayPal.
Take care,
-Chris