SAN FRANCISCO -- ChannelAdvisor said on Tuesday that client sales growth on Amazon's and eBay's (EBAY) online marketplaces slowed in December.

ChannelAdvisor reported that client sales on Amazon.com grew 29.8 percent in December, compared with the same month last year. That year-over-year growth rate was down from November's rate of 43.7 percent.

ChannelAdvisor also reported that client sales on San Jose-based eBay.com rose 22.2 percent in December, compared with a year earlier. November's growth was 27.4 percent.

ChannelAdvisor helps merchants sell more on websites such as Amazon.com, eBay.com and Google (GOOG) Inc's shopping site. Investors watch the firm's data closely because it provides an early window into the performance of the leading U.S. e-commerce companies.

The December results suggest a slowdown from Thanksgiving and the days following, which include crucial holiday shopping days known as Black Friday and Cyber Monday.

For the five-day period from Thanksgiving through Cyber Monday, which ChannelAdvisor calls the "Cyber Five," client sales on eBay.com rose 38.3 percent compared with the same days in 2011. Client sales on Amazon.com jumped 37.7 percent over that five-day period.

The holiday shopping seasons online got off to a strong start, but weakened after that. Online spending grew 14 percent for the whole season, compared with an estimate of 16 percent before the season began, according to comScore.

ComScore blamed the sluggishness on weaker consumer confidence declines fueled by prolonged fiscal cliff negotiations by U.S. politicians to avoid tax hikes and spending cuts that many feared would cause a recession.