First up is online auction house eBay ( symbol). eBay's namesake auction platform owns around 15% of all the world's e-commerce volume, an impressive feat considering the more hands-on nature of buying or selling on eBay versus a more traditional online retailer. There's considerable value in the ability to pair buyers and sellers -- and eBay has built itself a seemingly impenetrable moat for providing that service.

But auctions aren't eBay's biggest focus anymore. That's because the firm makes up nearly half of its sales through its online payment arm, PayPal. At first, PayPal was little more than a way for eBay to keep auction payments in-house. Today, though, the firm is working hard to make PayPal a relevant offline payment method that's used at brick-and-mortar retailers too. Already, the firm has made some major inroads, with a new deal with Discover Financial Services (DFS) that will dramatically increase the usability of PayPal's digital wallet product.

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All of that said, PayPal has earned a reputation for being difficult to deal with and locking up customer or merchant funds for months at a time. If the firm wants to keep growing PayPal (and taking home big fees for its trouble), then it'll need to resolve those customer service issues to be more in line with other payment networks.

In the meantime, the growth trajectory of both the auction businesses, and PayPal is indisputably upward.

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