Yahoo! News - EBay's Secret Weapon

By research firm Gartner's reckoning, PayPal services 27 percent of adult Web users who buy items online, putting it far ahead of Yahoo's Directpay, Western Union's BidPay and Citibank's c2it.

Generally, the auction giant's sellers sign up for a premiere or business account with PayPal, for which they pay 2.9 percent of the transaction value, plus 30 cents.

In contrast, other payment services traditionally have charged buyers rather than sellers -- and thus have gained less momentum among e-shoppers. For example, Western Union's BidPay charges buyers $5 for every $100, plus 2.25 percent for every dollar above that. c2it charges 2 percent per transaction. Both services place limits on the amount a buyer can transact.