Roughly 2 billion items a year pass through eBay, and the company is looking to outside partners and ISVs to help push that number even higher.

The Internet auction Goliath now counts 30,000 members in its developer program, and a quarter of its listings come through third-party tools tapping into its Web services interfaces. But eBay wasn't always so eager to open its platform and build a channel.

Just ask Vendio. The first time that the eBay Certified Solution Provider tried to build tools to tap into the eBay marketplace, it nearly got sued.

"There was a fair amount of saber-rattling," Vendio CEO and co-founder Rodrigo Sales said, reflecting on Vendio's early days in 1999. Then doing business as AuctionWatch, Vendio created a site that scraped listings from various auction Web sites, including eBay, and offered a universal search. EBay called the service an illegal infringement on its platform and ordered AuctionWatch to stay away from its listings. The two companies exchanged legal threats.

Two years later, Vendio signed on as eBay's first partner in its "preferred solution" program. The about-face came when eBay realized that outside partners would generate more business for its auctions than they pulled away.

"Anything that helps accelerate trading is good for our users and good for our market," said Greg Isaacs, who joined eBay in 2001 and now serves as director of its developers' program. "We want to be an online trading platform where anyone can trade anything, and our developer program is a critical part of that."

EBay's epiphany came in 2000, when it surveyed the growing landscape of startups scraping its listings and cobbling together tools for eBay sellers. It decided to join the throng rather than fight it. EBay released its first open API beta test in 2000 and soon after created a formal program to encourage outside application development around its platform.

The program has since snowballed. In 2002, eBay processed about 1 billion calls to its Web services interfaces. In the first quarter of this year alone, it processed nearly ten times as many.

EBay is like a gravity well in the world of e-commerce. The total value of items sold through its Web site reached $44.3 billion last year, a 30 percent increase over the previous year's total. A sales stream that large offers plenty of opportunities to operate profitably in eBay's orbit, partners say.

"The ability to turn your infrastructure on and plug into that marketplace, that's a siren song," said Eric Smith, founder and president of UnWired Buyer, an Austin, Texas-based mobile bidding software maker. "To date, 100 percent of our focus is on eBay."

UnWired Buyer was the winner of eBay's first "developer challenge," a competition to create a killer app. Its service allows users to bid by mobile phone in the final minutes of an eBay auction.

"I can't tell you how many auctions I've won using [UnWired Buyer's] application," said eBay's Isaacs, who confesses to doing much of his shopping for household goods on eBay. His recent purchases include an air mattress for his visiting in-laws.