WordPress.com has changed their policies a couple of times on theme submissions. It was closed, then it opened up, and now it’s closed again.
Sarah Gooding did the hard work on this one:
I contacted Automattic to find out why the submission form has been removed and when authors can expect for it to be re-opened. The company’s official statement indicates that opening up the marketplace was a temporary experiment and that there is no ETA for relaunching it.
They will apparently go back to the invite system. This is part of a pattern; they also no longer blog about commercial themes from third parties.
I’d love to see more open data on the WordPress.com side of things with regard to themes. Sellers have some info, because they know their own sales and how their themes rank on the platform, but that’s about it.
From what many sellers have noted, while it’s a decent place to put a theme, it’s not where you’ll make the majority of your income, and the review process takes forever on top of that.