Web developers everywhere might be breathing a collective sigh of relief today as eBay opened its APIs, which it says will simplify the job of creating add-ons for the online auction site and integrate it with enterprise applications. But perhaps more valuable to some is the access gained to the large community of sellers—currently at about 700,000—many of whom themselves have to purchase their wares from somewhere.

The news of the so-called Project Echo came at the eBay Developers Conference in Chicago, which runs through Wednesday. An open beta program is set for early next year.

Major retailers such as Sears and others have for years used eBay as an outlet for new, irregular, discontinued or refurbished goods, attracting buyers in search of bargains. APIs published through the eBay Web Services program permit the merchants to tie eBay.com to their own back-end systems, in essence using the Web-based auction software as a retail front-end.

Such applications account for more than a quarter of eBay’s listings, according to the company. EBay’s auction software, known collectively as the Selling Manager, has until now been proprietary. What’s new is that sellers that have developed their own tools will now be able to offer those customizations to other sellers through Selling Manager, in theory creating new revenue opportunities for sellers and developers.

The company also announced that it will revamp and renew the Developer Portal for PayPal, its online payment system. Set to launch in July, new services in the portal will include sample code, sales-lead generators, marketing tips and case studies, all designed to help sellers boost sales. PayPal also will expand its 25 APIs to simplify selling to repeat customers and introduce subscription billing.