EBay has been trying to move away from its online auction roots, emphasizing new items selling at fixed prices to better compete with Amazon.
"The numbers suggest they're having success reintroducing consumers to the 'new eBay,'" Hottovy said.
PayPal, the payments division of eBay, said the volume of mobile transactions it processed by 2:00 p.m. New York time on Cyber Monday almost tripled versus the same period last year.
Client sales on Amazon.com were up 52 percent during the first part of Cyber Monday, ChannelAdvisor also reported.
"Amazon continues to look impressive to us since it is building on top of large numbers," said Scott Tilghman, an analyst at Caris & Company.
Online retailers held back some of their best promotions and biggest discounts until Cyber Monday, which helped spur sales, IBM's Henderson and ChannelAdvisor's Wingo said.
Amazon offered $30 off its 7 inch Kindle Fire tablet, which usually sells for $159. The deal was only available on Cyber Monday and was still available at 5:00 p.m. New York time.
EBay promoted Cyber Monday deals on iPads, made by Apple, and Nook devices from Barnes & Noble.
These types of discounts attract shoppers to Amazon and eBay's websites, where they may purchase other items too, Wingo explained.
'Wii U' Sells Out
EBay has also benefited as some hot holiday items sold out this year at some retailers. When that happens, shoppers often turn to eBay, where third-party sellers are usually still offering the items at higher prices.
Nintendo said on Monday its new "Wii U" video game consoles sold out at retailers in the United States.
The devices were still available on ebay.com on Monday at 10 to 20 percent above the suggested retail price, according to Jesse Divnich, an analyst at video game research firm EEDAR.
Despite strong sales data, analysts are concerned that heavy discounting may pressure retailers' profit margins, online and offline.
The average online order size on Cyber Monday was $130.30 as of 3:00 p.m. New York time. That was down from almost $200 during the whole of Cyber Monday last year, according to IBM.
Online order sizes are shrinking as consumers buy more digital goods, such as e-books, music and video, which generally cost less. However, discounting is also pressuring order size and that could feed through to lower margins, Morningstar's Hottovy said.
EBay margins should be relatively well protected because the company charges a commission on sales by third-party merchants and retailers.
Amazon operates like this, but, unlike eBay, the company also has its own product inventory so it may be exposed to margin pressure, according to Colin Sebastian, analyst at R.W. Baird.
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