ChannelAdvisor Corp.’s clients that sold products on online marketplaces increased their sales 16.2% during the holidays. That would exceed the online holiday sales grow estimate of 13.9% from IBM Corp.’s Digital Analytics Benchmark, and be in line with a comScore report today of growth of 15% when just counting purchases on computers, not mobile devices.

ChannelAdvisor produces monthly reports that compare its merchant clients’ sales through various online shopping portals and marketing channels, including Amazon.com, eBay Inc. and Google Shopping.

Amazon’s same-store sales, which ChannelAdvisor defines as merchants who have sold on the marketplace for at least a year, grew 26.9% during the November and December holiday season. Amazon, No. 1 in the Internet Retailer 2014 Top 500 Guide, started the season with aggressive promotions, which helped ChannelAdvisor clients’ sales jump 35.7% year over year in November. The growth rate slowed to 21.8% in December.

Moreover, 44% of Amazon’s gross merchandise volume sold by ChannelAdvisor clients during December was shipped through Amazon’s Fulfillment by Amazon program, a service that lets merchants store inventory at Amazon warehouses with Amazon handling shipping to the customer. That’s up from 40.9% in December 2013.

In addition, some ChannelAdvisor clients are letting Amazon deliver orders they take on their own web sites. The percentage of gross merchandise value sold by ChannelAdvisor clients shipped through the Fulfillment by Amazon program that was not sold through Amazon’s Marketplace inched up to 1.1% in December, a slight increase from 0.9% a year earlier.

But while marketplace sellers’ sales on Amazon surged, revenue increased more modestly for those selling on eBay. eBay’s same-store sales grew 7.3% during the two-month holiday season compared with a year earlier. In December, eBay’s same-store sales grew 5.8%, down from a 9.8% increase a year earlier. eBay’s fixed-price sales grew 5.7% in December, down from 9.9.% growth in November.

ChannelAdvisor also reported same-store sales for comparison shopping engines grew 12.3% year over year during November, but fell 2.3% in December. That compares to Google Shopping, which grew 22.5% in November compared to a year earlier and 13.2% in December. Moreover, ChannelAdvisor’s clients’ Google Shopping conversion rates rose to 3.61% in December compared to 3.29% a year earlier and their average order value grew 23.8% to $114.24 from $92.31.