The program allows for developers to build sites that drive transactions on eBay, and then get paid by eBay. Or, developers can build software that is used by sellers as front ends to eBay, whereupon developers are paid by sellers. About 22 percent of eBay listings are generated though these third-party applications.

Fees have been charged for using APIs to access the program. "Essentially for us, we really wanted to build on the past success we've had and we looked at what are the barriers to innovation on the eBay platform and the cost was one of those barriers," said Greg Isaacs, director of the eBay Developers Program

The free API usage applies to developers using the unified schema eBay announced last February. API calls made using legacy schema will be supported until next June and will continue to be charged at previous call rates. This provides an incentive to move to the unified schema, eBay said.

EBay has charged US$1.25 per 1,000 APIs calls as a starting rate, or annual fees starting at $500. But these go away for unified schema users, as do certification fees paid to eBay employees to verify the efficiency of applications before they go online. Developers also can promote their application in the eBay Solutions Directory, for third-party solutions designed to enhance selling on eBay.

Isaacs would not disclose how much revenue eBay would sacrifice by eliminating these fees, but he pointed out that this revenue has never been important to eBay; revenue from buyers and sellers is what counts.