The hub of all things WordPress, as in the kind you install on your own, is wordpress.org. There’s both a plugin and a theme directory there, and the latter has now gotten some commercial (aka premium) themes treatment. However, the new page merely lists some resellers of commercial GPL themes, so this is not the marketplace a lot of people has been waiting for.
Some of them you may pay for access, some of them are membership sites, some may give you the theme for zero-cost and just charge for support. What they all have in common is people behind them who support open source, WordPress, and its GPL license.
If you’re selling GPL themes for WordPress you can get listed, just scroll down to the bottom of the page. Personally, I think the themes should be hosted on wordpress.org so that they got automatic updates and so on, but that’s a whole other story. I do hope this is just the first step of many in this area, but we’ll see. An official blog post has yet to outline this addition, which might or might not have been up for some time, I really can’t tell.