With more than 2.25 million products, Amazon.com Inc.’s AmazonSupply.com has emerged as the biggest competitor in the eyes of many business-to-business e-commerce sellers.

But along with its rival eBay.com, B2B sales—the product category referred to as Business & Industrial, or B&I—are also giving Amazon’s and eBay’s own online retail sales stiff competition as the source of the top growth rate in sales, says Scot Wingo, CEO of ChannelAdvisor Corp., which helps online companies sell through Amazon, eBay and other e-marketplaces.

EBay Inc. no longer breaks out B&I growth rates, but in the fourth quarter of last year it noted that B&I sales were growing at an annual clip of 17%, compared with 12% overall for eBay.com, Wingo says. Although Amazon hasn’t broken out similar numbers, he figures that total B2B sales at AmazonSupply.com and Amazon.com are growing at rate the same or higher than Amazon.com’s overall sales. “My educated guess would be, with Amazon growing overall in the mid-20’s, B2B there is growing in the 25% to 35% range,” he says. EBay and Amazon did not immediately return calls for comment.

For 2013, Amazon reported revenue of $74.45 billion, up 21.9% from $61.09 billion in 2012. Its 2013 web sales, including commissions from retailers that sell through the Amazon marketplace but excluding revenue from Amazon Web Services and other units not directly related to retail sales, were estimated by Internet Retailer at $67.86 billion, up 20.3% from $56.41 billion in 2012. EBay reported the gross merchandise value of goods sold on its global marketplaces in 2013 of $76.50 billion, up 12.9% from $67.76 billion a year earlier.

AmazonSupply.com, meanwhile, is taking a B2B marketplace approach different from that of Amazon.com and eBay.com. Although designed primarily as retail e-commerce sites, Amazon.com and eBay.com each feature a B&I category, with dozens of subcategories like office supplies and industrial equipment. All items sold on eBay are by outside sellers; Amazon sells some B&I products on its own behalf, products it has purchased from manufacturers and distributors, and also offers products from marketplace sellers.

AmazonSupply.com, however, sells only products that it has acquired from manufacturers and distributors. “AmazonSupply is what we call first-party only,” Wingo says, “meaning that “to have your products there, you need a traditional wholesale business with Amazon, and Amazon is the ‘retailer/distributor.’”

AmazonSupply is also building its business in other ways. It pays up to 8% on referrals from other sites through its Associates program, which is also available to affiliate sites that refer visitors to Amazon.com.

Wingo, whose company has some 2,500 clients, including many that sell through e-marketplaces, says he generally hears that B2B products are growing faster than overall sales on eBay and Amazon. At the same time, he adds, “we’re definitely seeing a lot of interest” among companies considering selling products to Amazon for sale on AmazonSupply.com.

Amazon is No. 1 in the Internet Retailer Top 500, which ranks companies by their annual web sales.

For a free subscription to B2BecNews, click here.