You could checkout http://appbodega.com http://appbodega.com. It's an App Store alternative and works as a standalone app that you need to install on your Mac.

You could checkout http://appbodega.com http://appbodega.com. It's an App Store alternative and works as a standalone app that you need to install on your Mac. You can sell your apps via Bodega as a developer. Though you cannot "Buy and Sell" apps. I think most licenses for apps would normally limit resale of apps to another buyer. So something on the exact lines of flippa.com may not be possible due to license restrictions.