In an attempt to bolster its online payment system, eBay today announced that it has agreed to buy Braintree, a full-stack payments platform that allows companies to accept payments in mobile apps and websites. The deal will cost eBay $800 million in cash, and once completed, the online auction site plans to integrate Braintree with its PayPal subsidiary."Braintree is a perfect fit with PayPal," said eBay Inc. President and CEO John Donahoe. "Bill Ready and his team add complementary talent and technology that we believe will help accelerate PayPal’s global leadership in mobile payments. Together, we expect that PayPal and Braintree also will accelerate our leadership in supporting developers who are creating innovative solutions for next generation commerce startups."Once acquired, Braintree will continue to operate as a separate service within PayPal under Braintree's chief, Bill Ready, who will report to PayPal president David Marcus. All of Braintree's staff, including management, are expected to stay in place, eBay said.The big score here for eBay is Braintree's Venmo app for mobile devices. Venmo makes it easy to make payments from mobile, which is a hot sector right now. PayPal's presence in mobile is already strong, and Venmo will contribute to its capabilities, according to eBay.