(REUTERS) -- Cyber Monday was on course to set another record for retail sales on Monday, as the strong start to the holiday shopping season continued online.

Online sales as of 6 pm Eastern time in the U.S. were up 15 percent versus the same period last year, IBM Benchmark, a unit of International Business Machines, said.

ChannelAdvisor, a software company that helps retailers sell more online, said its clients saw same-store sales rise 40.2 percent during the first half of Cyber Monday, versus the same period a year earlier.

Cyber Monday is traditionally the first Monday after Thanksgiving when employees return to offices and purchase items with their work computers.

Consumers are responding to retail promotions and are doing a greater percentage of their holiday shopping from their desk, said Eric Best of e-commerce firm Mercent.

Retailers that have been Mercent clients for at least a year saw online sales jump 37 percent during the morning of Cyber Monday, compared to the same period last year.

It's better than we were expecting, Best added. But this may be a shift to people buying earlier, rather than an increase in the total volume of online sales for the whole holiday shopping period.

In 2010, Cyber Monday sales topped $1 billion, making it the heaviest day of online spending ever, according to comScore Inc. This year, the firm expects Cyber Monday sales to exceed last year's tally.

This will be the biggest day for retail sales online ever, said John Squire of IBM's Smarter Commerce initiative. Retailers are doing a great job promoting and discounting to consumers on the days they want to shop.

AMAZON

ChannelAdvisor said clients saw same-store sales via Amazon.com surge 51.4 percent during the first half of Cyber Monday, compared to a year earlier.

Amazon has done a lot of new promotions and emailing to its customer base this holiday season, which has increased visits to the company's website, Scot Wingo, chief executive of ChannelAdvisor, said.

Interest in the company's new Kindle Fire tablet computer has also driven consumers to the website, he added.

Amazon.com had 50 percent more visitors on Black Friday than any other retailer, according to comScore data.

The site is very good at recommending other products to customers once they're there, Wingo said.

Amazon shares rose 6.4 percent to close at $194.15 on Monday.

EBAY

ChannelAdvisor clients saw same-store sales via eBay's online marketplace climb 17.1 percent during the first half of Cyber Monday, versus the same period last year.

Mercent clients saw same-store sales via eBay more than double early on Cyber Monday, compared to last year.

That was partly driven by new sellers on eBay that are growing quickly from a smaller start, Best said.

However, eBay has improved its marketplace this year to help fixed-price sellers, rather than the auction sellers for which the company used to be known, Best added.

Earlier this year, eBay introduced a new online checkout basket that lets shoppers purchase multiple products from different sellers at the same time, Best explained.

The eBay shopping experience is becoming more Amazon-like every day, Best said.

EBay shares rose 5.1 percent to close at $29.66 on Monday.

(Reporting by Alistair Barr; editing by Carol Bishopric)

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