eBay is opening its site so third-party developers can integrate applications directly into eBay Selling Manager, an online tool for managing and tracking listings on eBay.

It’s excepting a flurry of sales optimization apps.

It says 700,000 professional eBay sellers subscribe to Selling Manager, which has previously consisted entirely of applications developed by eBay in-house.

Third-party applications built using eBay Web Services account for roughly 28% of all eBay.com listings.

eBay is promising independent developers tighter and more lucrative integration with the site. It said finding and subscribing to a third-party application or feature in Selling Manager will be identical to how eBay-developed widgetry is offered on eBay.com.

Once selected, the application will be included in a user’s set of advanced selling features from My eBay. Interested subscribers will get a free 30-day evaluation period to trial applications.

Developers have to meet site standards for trusted buying and selling experiences. Once approved, they have a shot at subscription-based revenue.

eBay’s Developers Program reportedly has more than 70,000 members.

See http://developer.ebay.com/echo.

Meanwhile, eBay’s little friend PayPal will be opening a new Developer Central portal (www.paypal.com/devcentralbeta) next month, designed to help developers support what it called growing merchant demand for online payments.

The portal is supposed to offer 25 APIs for sending and receiving money online, making refunds, managing subscriptions and tracking transaction histories along with pre-packaged sales presentations, customizable datasheets and case study collateral to help developers sell integration projects; best practice advice about responding to requests for proposals (RFPs) and tips on effective marketing; sample code; training and education materials about online payments; and forums to exchange ideas with the PayPal team and fellow developers.

PayPal has also introduced a new Recurring Payments API expanding subscription billing, and a new Reference Transaction API to support repeat customers. See www.paypaldeveloper.com/pdn/blog?blog.id=devblog.

PayPal claims to have 35,000 active developers and 300 Certified Developers in its Developer Program.