Just like any other APEX Application you can (probably) still export that application and install it on your own environment. But they will ne "locked down", although it's not quite clear what that actually means. Probably, to prevent support issues due to your own changes, you aren't allowed to make changes. BTW, not all Markterplace Applications will be available on OTN, some will, others won't ..
Another, not yet implemented, idea is that you can also upload your own application into the Marketplace. The application will be reviewed by Oracle before it's made available. Your application won't actually be up there, but an interested user will be redirected to your own site and be able to download it from there. And the idea is that you can either provide these applications for free or charge for it. Don't expect it to make you a millionaire, but, hey, that should go dollar-by-dollar anyway...
As a teaser a screenshot of the applications that will be available from the start:
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