It's official! eBay's (EBAY -1.13%) PayPal has begun going live at over 2 million U.S. retail locations where Discover Cards are accepted. This is a pinnacle moment for the online payment processor that's trying to capture share in the tremendous point-of-sale market. Despite eBay's ambitions, there are many challenges for the company to overcome before investors can call this move a success. In this video, Motley Fool contributor Steve Heller examines how big of an opportunity this could be for PayPal and what factors will contribute to its success.