EBay Inc. and Amazon.com Inc. sales growth slowed in September, signaling that consumers may not increase holiday spending as much as expected, said Herman Leung, an analyst at Susquehanna Financial Group.

EBay’s comparable sales rose 22 percent in September from a year earlier, less than August’s 29 percent growth, ChannelAdvisor Corp. said Tuesday on its website. Amazon’s growth rate fell to 37 percent last month from 41 percent in August, the Morrisville, North Carolina-based company said. Comparable sales measure revenue for merchants who have used the site for at least a year.

“This holiday season could start off slow,” said Leung, who’s based in San Francisco. “September is a preview. The confidence level has gone down a little bit, looking at this data.”

New advertising options from Google Inc. that let merchants include more product information in search ads -- such as prices and images -- may be worsening the decline, Leung said. The ads could be filtering buyers to the direct sellers doing the advertising instead of e-commerce companies such as Amazon and EBay. Consumers also may be paring back spending as they prepare for the holidays, he said.

Shares of EBay, based in San Jose, California, fell 3.6 percent to $46.22 Tuesday at the close in New York, while Amazon dropped 3.1 percent to $250.96.

Amazon, based in Seattle, is increasingly adding third-party sellers to its site and, at the same time, rolling out proprietary Kindle e-readers and tablets to woo consumers with its own products. Those sales contributed to total e-commerce spending that rose 15 percent in the second quarter from a year earlier, according to research firm ComScore Inc.

EBay is moving away from auctions and focusing more on fixed-priced sales to drive growth. Auctions declined 17 percent in September from the previous year, while fixed-price sales rose 24 percent, according to ChannelAdvisor. ChannelAdvisor’s software helps third-party sellers manage online sales on e-commerce sites.