EBay's ambitious project to remake its Selling Manager set of e-commerce tools into an open platform and repository of third-party applications reached another milestone on Monday when it was opened to the marketplace's sellers.

Merchants can now sign up for a free subscription to the new Selling Manager and give external applications that interest them a test run, since all fee-based applications have free trial periods, Dinesh Lathi, eBay vice president of Seller Experience, said in a blog post.

"From sourcing product to shipping your items, Selling Manager Applications are designed to make your business easier to run," Lathi said.

Selling Manager has been around for years, amassing a user base of about 700,000 merchants, but it only featured applications built by eBay.

In parallel, eBay has had a program for external developers since 2000 that has been joined by about 96,000 developers who have created about 13,000 applications.

Then in June of last year, eBay announced its intention to turn Selling Manager into an application repository and platform open to external developers.

After a months-long closed pilot, eBay opened the new Selling Manager to all developers in April of this year and promised it would make it available to merchants at some point this summer.

With the new Selling Manager, eBay hopes to boost developer participation in the eBay platform, while helping merchants more easily discover and access useful applications.

The new Selling Manager is launching with about 12 applications, a number expected to grow steadily in the coming weeks and months, said Kumar Kandaswamy, head of eBay's developer program and platform product, in an interview.

"We'll have constant growth of applications as part of this platform. We have a rigorous vetting process to make sure the applications meet the needs of sellers both in terms of scale and of functionality," Kandaswamy said.

In addition to being an application platform and repository, Selling Manager is also a store, since eBay will handle billing on behalf of third-party developers, he said.

As compensation for its Selling Manager services, eBay will keep 20 percent of the revenue from applications, while developers take the other 80 percent, he said. The new Selling Manager is currently available in the U.S. only.